Unfortunately, I cannot explain what might have happened.
You should certainly be able to edit a gallery without losing any gallery images. I've not encountered this problem before (and I don't recall any other users reporting such a problem).

It certainly sounds like the image entries were removed from the gallery's 'config.xml' file. If you upload a modified gallery folder to your website, then existing images will not be deleted (but if the image entries are no longer in the 'config.xml' file, then the images will no longer be displayed in the gallery).

If the problem happens again, check your gallery's 'config.xml' file (you can open it in a plain text editor) to see if the <image> tags for all your images are present.
Also, before saving your gallery on the 'Publish' tab, double-check that all the images are present on the 'Images' tab.

This is also very strange (although, as you say, if the 'images' folder permissions are too restrictive, then the browser will not have access to the image files).

Re-uploading images to your website should not change their permissions.
I do not know how you are uploading your gallery files but, in case it helps, I use Filezilla (a free dedicated FTP program) which I've used for many years without issue.

If you find that re-uploading a file (or folder) to your web server changes its permissions, then you might like to try a different FTP program to see if this makes a difference. An alternative FTP program for Windows is WinSCP (also free).

I'm not sure if these notes will help but if you can replicate the problem consistently, please post back with a set of steps that I can follow to try to reproduce the problem myself and hopefully help further. Thank you.

As you have discovered, JuiceboxBuilder-Pro does not use the EXIF orientation flag embedded within images. When images are added to a gallery, the EXIF orientation flag is not read and, when images are resized for galleries, all metadata is removed from the images.

If you need metadata to be retained in gallery images, you could resize your images in an imaging program such as Adobe Photoshop (retaining metadata during the resizing process) before feeding them to JuiceboxBuilder-Pro and deselect the 'Resize Images' checkbox so that JuiceboxBuilder-Pro does not process the images further.

However, this still might not be enough to solve orientation issues. It can be difficult to know exactly how a certain program will display an image with an EXIF orientation flag and there seems to be little consistency across different programs (and web browsers).

For example, try viewing this image directly in different browsers.
http://7stripes.esy.es/sample/image001.jpg

In Edge and IE11, the image is displayed in landscape orientation (the EXIF tag not being read).
However, in Chrome, Firefox and Opera, the image is displayed in portrait orientation (correct according to the EXIF orientation tag which I changed by rotating the image in Windows File Explorer).

When the image is displayed by an HTML <img> tag (just as Juicebox does in a gallery's web page), all browsers (at least the ones I listed above) do not seem to respect the EXIF orientation tag and just display the image as it is.
http://7stripes.esy.es/sample/index.html

You could try including the image-orientation: from-image; CSS rule in your gallery's web page (applied to all 'img' tags) but browser support is currently limited (to Firefox 26 and later) and Juicebox itself would not know that the image had been rotated by the browser which may result in the image being displayed in the gallery with unexpected dimensions (the drop-shadow around the image may be in an incorrect position).

With all this in mind, perhaps the best way to ensure that your images are displayed as expected in your gallery would be to re-save your images in an imaging program (such as Adobe Photoshop), stripping out the metadata during the resizing process and then rotate your images so that they are visually oriented correctly (inverting their actual aspect ratios rather than introducing new EXIF orientation flags) before feeding them to JuiceboxBuilder-Pro.

EXIF orientation can be a minefield at times and, as can be seen above, it is not always easy to predict how an image will be displayed in any given program.
This article, whilst a little out-of-date, makes for interesting reading.

Knowing how JuiceboxBuilder-Pro works should at least allow you to adjust your workflow to obtain a gallery with correctly orientated images.

If you do not have Adobe Photoshop, then there are other imaging programs you could use to batch process (re-save with or without metadata, rotate, etc.) your images, such as Irfanview. (This is just a personal suggestion rather than an official recommendation.)

I hope this goes some way towards explaining things.

I do not know how likely it is that JuiceboxBuilder-Pro's behavior will be changed in the future but, if you like, please feel free to make suggestions for future versions in the Feature Requests forum thread.
This keeps all the ideas together and ensures that they are not overlooked by the developers.
Thank you.

@gfs

I'm glad to hear that just deleting the 'ELS' folder worked for you, too.
Thanks for sharing!

For the Back Button to appear in Small Screen Mode, you'll need to set showSmallBackButton="TRUE" (in the Back Button section).
Please note that in Small Screen Mode, the Back Button will be displayed on the main image pages but not the thumbnail pages.

If you really need a link on the Small Screen Mode thumbnail pages, then you could use HTML formatting within the Gallery Title (as noted in this FAQ) to include an <a> tag. For example, you could enter a galleryTitle (in the 'Lite' options section) such as:

Click <a href="http://www.example.com/index.html">here</a> to return to main page.

If you choose to use display a Gallery Title in your gallery, then it will be displayed in Large Screen Mode and on the thumbnail pages (but not the main image pages) in Small Screen Mode.
You can position the Gallery Title (for Large Screen Mode) using the galleryTitlePosition and galleryHAlign configuration options (in the 'General' options section).

I thought this might be the case and mentioned it in a post above:

There really is no way to allow right-clicking to save an image in a Juicebox gallery and, if you want to allow users to be able to download images from a Juicebox gallery, the best options would be to display the Open Image and/or Download Button on the gallery's Button Bar (via showOpenButton="TRUE" and/or showDownloadButton="TRUE").

Unfortunately, it looks like it's just not possible to copy text from within a Juicebox gallery (across all browsers) and there's no easy way around this.
If it is really important for visitors to your website to be able to copy and paste image titles and/or image captions, you could use the Juicebox-Pro API to fetch the image title and caption for the currently displayed image and use JavaScript to display the text elsewhere on the web page (in a separate <div> container outside the gallery) where it can be copied with ease.
Here's as example of how this can be achieved.

<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="utf-8" />
        <meta name="viewport" id="jb-viewport" content="width=device-width, initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0, user-scalable=0" />
        <style type="text/css">
            body {
                margin: 0px;
            }
        </style>
        <script type="text/javascript" src="jbcore/juicebox.js"></script>
        <script type="text/javascript">
            var jb = new juicebox({
                containerId: "juicebox-container",
                galleryHeight: "400",
                galleryWidth: "600"
            });
            jb.onImageChange = function(e) {
                var index = e.id;
                var info = jb.getImageInfo(index);
                var title = info.title;
                var caption = info.caption;
                $('#text').html('<p>Image Title: ' + title + '</p><p>Image Caption: ' + caption + '</p>');
            };
        </script>
        <title>Test</title>
    </head>
    <body>
        <div id="juicebox-container"></div>
        <div id="text"></div>
    </body>
</html>

On your 'decBirds&Flowers.asp' page, when the page is initially loaded, I can right-click on your 'Birds & Flowers' gallery.
If I then switch galleries (to your 'Tiles on the Wall' gallery), the right-click functionality is still available.
It's only when I change pages (for example, to your 'decFarmyard.asp' page) that the right-click functionality disappears (and that's because the re-enabling code is not present in those other gallery pages).
If you find that switching galleries on the same page disables the right-click functionality, then try replacing the following code in your loadGallery(a, b, c, y, z) function:

if (tracker === false) {            
    jb.onInitComplete = function() {
        var element = document.getElementById('juicebox-container');
        element.oncontextmenu = null;
    };
}

... with this alternative (which should re-enable the right-click menu for all elements on the web page every time a gallery is selected):

jb.onInitComplete = function() {
    var elements = document.getElementsByTagName('*');
    for (var i = 0; i < elements.length; i++) {
        elements[i].oncontextmenu = null;
    }
};

Even though it's not possible to right-click and save an image or highlight and copy text from within a Juicebox gallery, I hope my suggestions and workarounds are useful (and thanks for posting your suggestions in the Feature Requests thread).

It looks like you have placed the code in the correct place in your 'index.html' file so it should work fine.

I have just double-checked my suggestion and it seems to work OK for me.
I created a multi-gallery album with jAlbum 15.1.3 and the Juicebox skin v1.5.1, giving each gallery an audioUrlMp3 and setting playAudioOnLoad to TRUE.

Without any modifications, the audio tracks play over each other when new galleries are selected from the side menu (as expected).
After adding the code:

if (jb) {
    $('.jb-bb-btn-audio').each(function() {
        if ($(this).hasClass('jb-status-playing')) {
            jb.toggleAudio();
        }
    });
}

... at the very beginning of the loadContent(galleryPath, galleryCount) function, the audio track from the current gallery stops before the audio from the newly selected gallery starts (again, as expected).

Try clearing your browser's cache to be sure that your browser is using your updated 'index.html' file.
I have just viewed the album page you posted in your original post above and it seems to work OK. I can clearly hear the audio from one gallery stop before the audio from a new gallery starts.
Please remember that the audio tracks are linked to individual galleries and there is no easy way to have a single audio track play (and continue to play) whilst switching between different galleries. (You'd need to manually embed a third-party audio player into your 'index.html' file to do this.)

Try the following. Replace your current loadgallery() function with the following code. You can use your existing tracker variable to ensure that the right-click removal code is run just once (and not each time a new gallery is loaded).

function loadGallery(a, b, c, y, z) {
    var jb = new juicebox({
        backgroundColor: "rgba(255,255,255,1)",
        thumbShadowBlur: "rgba(255,255,255,1)",
        imageShadowColor: "rgba(255,255,255,1)",
        textShadowColor: "rgba(255,255,255,1)",
        captionBackTopColor: "rgba(0,0,0,0)",
        containerId: "juicebox-container",
        baseUrl: y,
        galleryHeight: z,
        maxThumbColumns: a,
        maxThumbRows: b,
        thumbsPosition: c
    });
    if (tracker === false) {            
        jb.onInitComplete = function() {
            var element = document.getElementById('juicebox-container');
            element.oncontextmenu = null;
        };
    }
    tracker = true;
}

This should work fine. (I tested it myself before posting.)

You're welcome.

Just for clarification, when trying to view a gallery locally in Chrome, the "Juicebox can not display locally in this browser." message should be displayed (as noted in the FAQ below), not the "Config file not found." message.
When I view my gallery locally, I see the message "Juicebox can not display locally in this browser". Why?

If you continue to experience difficulties, just let me know and I'll do my best to help you out.

Incidentally, if you are still using Juicebox-Pro v1.5.0, you might like to upgrade to the latest version (v1.5.1) as many bugs have been fixed in the latest release. (Please see the Version History for a full list of changes.)
Upgrades are free within the same major version number and download links always point towards the latest version rather than the version you purchased so you could use your existing download link (from your purchase email) to download the latest version.
If you cannot find your purchase email or your link has expired, then just fill in the Download Link Request Form and I'll send you a new link.
Full instructions for downloading the latest version and for upgrading JuiceboxBuilder-Pro and existing galleries can be found on the Upgrading Juicebox support page.

If you encounter the "Config file not found." message whilst trying to view a gallery locally in Safari 11, then please see this forum post for a full explanation and a workaround.

Unfortunately, Safari 11 (released 19 September 2017) introduced a new security restriction which prevents local viewing of Juicebox galleries by default. This issue is specific to viewing galleries locally (from your computer's hard drive) and there is no such issue once the galleries have been uploaded to a web server (where they can be viewed in any modern browser).
Selecting 'Disable Local File Restrictions' from Safari 11's 'Develop' menu (full instructions in the link above) should, once again, allow you to view your galleries locally in Safari 11.

If you are not using Safari 11, then please see this FAQ for more general information on the "Config file not found." message:
When I view my gallery I see the message 'Config file not found'. How do I fix this?

I hope these notes help.
However, if you continue to experience difficulties, please let me know and I'll try to help further.
Let me know if you encounter the error whilst trying to view a gallery locally or after the gallery has been uploaded to your web server. Also, let me know what browser(s) you see the message in and, if your gallery has been uploaded to your web server, please post a link to the gallery so that I can see the problem for myself.
Once I'm able to see the gallery on your web server, I should hopefully be able to determine the cause of the problem and propose a solution.

No problem!

Unfortunately not. Dropbox is a file sharing service rather than a regular web host.
If you uploaded an image to Dropbox, you'd need to be able to just view the image on its own but Dropbox wraps the image within its own 'download this image' web page and it is not possible to just view the image in isolation.

Other than hosting the images on your own web site, you could use a Flickr account as a source of images for a Juicebox gallery. You could upload your images to a Flickr account and then select which images you want to display via Juicebox's Flickr options (see the Lite and Flickr Pro sections in the Config Options page).

Exactly the same plugin (with the same scaling routine) is used for both Photoshop CC 2017 and Photoshop CC 2018 so, assuming the settings and source images are the same, then I would certainly expect the output to be the same, too.

I see that the 1st image in your test gallery is larger (1500 x 1000) than the same image in your 'Chili Night' gallery (1024 x 683) so Juicebox has to dynamically resize the image to a different extent in both galleries when they are displayed.

Please double-check the 'Maximum Image Width, px:' and 'Maximum Image Height, px:' values in the script's interface in both Photoshop CC 2017 and Photoshop CC 2018 to make sure that you are using the same values in both programs.

If the source images are the same, the plugin is the same one and the setting are identical then the only other variable is Photoshop itself but I really cannot explain why CC 2017 and CC 2018 would be producing different results.

If you find that you are not getting the results you like from Photoshop CC 2018, then you might like to try creating your galleries by a different method, such as JuiceboxBuilder-Pro or the Juicebox Lightroom Plugin (if you have Adobe Lightroom).
I realise that this might not be as convenient for you as using Photoshop and you might need to change your workflow a little but if the results are improved, then it might be worthwhile.