Did you try adding all 100 images at once from the shallow directory structure (and, if so, did it work)?
There is a known issue which can occur when adding many images at once and if you are adding a large batch of images, the maximum path length might just be a red herring.

If you add many images at once, Adobe AIR may not be able to allocate enough memory for the task at hand. Even if your computer has plenty of free RAM, Adobe AIR applications are limited to around 1GB of memory allocation. The workaround is to add your images in smaller batches.

I don't know if this is relevant but it might be something to bear in mind when testing further.

It sounds like the problem is due to an underlying maximum path issue with either Windows 10 or Adobe AIR (the platform on which JuiceboxBuilder runs).
Either way, I think there is little that can be done (other than moving the images to a more shallow directory structure before feeding them to JuiceboxBuilder). I'm fairly certain that JuiceboxBuilder is not imposing a maximum path length itself.

Your testing sounds fairly conclusive: fails with a long path; works with a short path.

I've tried to replicate the problem myself but, strangely enough, have not been able to.
I created a directory structure as long as Windows 10 will allow (using the alphabet as a folder name and topping things off with a couple of digital at the end to max it out).

C:\abcdefghijklmnopqrstuvwxyz\abcdefghijklmnopqrstuvwxyz\abcdefghijklmnopqrstuvwxyz\abcdefghijklmnopqrstuvwxyz\abcdefghijklmnopqrstuvwxyz\abcdefghijklmnopqrstuvwxyz\abcdefghijklmnopqrstuvwxyz\abcdefghijklmnopqrstuvwxyz\abcdefghijklmnopqrstuvwxyz01

I then copied a few images to the 'abcdefghijklmnopqrstuvwxyz01' folder, added them to JuiceboxBuilder from this location and the application created the gallery successfully.

Even setting the Gallery Folder on the 'Publish' tab to be the long path above, I cannot get JuiceboxBuilder to crash (under Windows 10 Pro v1703 x64) even though it fails to create all the gallery files in the 'abcdefghijklmnopqrstuvwxyz01' folder.

I think the resolution to this problem is just to use shorter paths.
However, you might like to try removing the default maximum path length that Windows 10 imposes to see if this makes a difference.
It involves either modifying the registry or making a change in the group policy editor. Please see this web page for details.
Please note that I am not recommending that you do this. It is just a suggestion if you are comfortable making changes to your Windows 10 system via the registry or group policy.
Please note that if you choose to make any changes via the registry or group policy, you do so at your own risk.
I recommend that you just shorten your paths before using JuiceboxBuilder.

Incidentally, I have not made this change to my own PC. My tests above were carried out using the default Windows 10 maximum path length.
Also, I realise that the path you posted above is less than Window's default maximum path length of 260 characters but removing this limit might be worth a shot if you feel you want to try something.

@JoergWenzel

Are you saying that JuiceboxBuilder-Pro processes your PNG images when the the 'Resize Images' checkbox is deselected?
This should not happen (and it does not happen in my own installation).

I've just double-checked and, with the 'Resize Images' checkbox deselected, JuiceboxBuilder-Pro v1.5.0 just copies the source PNG files across to the gallery's 'images' folder (but with .jpg file extensions) with no processing or resizing at all.
The files in the gallery's 'images' folder are identical (byte-for-byte) to the source PNG files. (I've checked their SHA256 hashes.)

As long as both the 'Resize Images' and 'Use Watermark' checkboxes are deselected, the source images should be copied across to the gallery's 'images' folder untouched.

Please try again and make sure that you deselect the 'Resize Images' checkbox (like in your screenshot) and save the gallery to a new empty folder (to ensure that pre-existing files are not present in the gallery folder).

If you find that images are still being processed when the 'Resize Images' checkbox is deselected, then I cannot explain why but it might be wise to try uninstalling and then re-instaling JuiceboxBuilder-Pro to see if this makes a difference.

Thank you for providing the URL to your gallery's web page.

It looks like the problem is that there is not enough room in your gallery's caption area to display the Flickr descriptions along with the Flickr titles.

When setting captionPosition="BELOW_IMAGE" (as your gallery does), Juicebox uses the maxCaptionHeight value as the actual height of the caption area (rather than a maximum value). (This is noted in the short description for maxCaptionHeight in the Caption Options section of the Config Options page.)

Your gallery currently sets maxCaptionHeight to "50px" which is too short to accommodate both the Flickr titles and descriptions.
Increase maxCaptionHeight (perhaps to its default value of 120) and you should see the Flickr descriptions below the Flickr titles.
(Incidentally, the value for maxCaptionHeight should be just a numeric value, without the 'px' suffix.)

I hope this resolves your problem.

You're welcome!
I'm glad you're getting on well with Juicebox-Pro.

First of all, please double-check to see if your gallery is using Juicebox-Lite (the free version) or Juicebox-Pro.
Juicebox-Lite has an image limit of 50 images per gallery whereas Juicebox-Pro has no such limit.
If your gallery displays a Juicebox logo in the lower right corner, then it is a Juicebox-Lite gallery. (Juicebox-Pro galleries do not feature this branding.)

If your gallery is a Juicebox-Lite gallery, then instructions for upgrading existing galleries can be found on the Upgrading Juicebox support page.
Essentially, you can load and re-save your gallery in JuiceboxBuilder-Pro or replace your gallery's existing 'jbcore' folder with the 'jbcore' folder from the Juicebox-Pro v1.5.0 download zip package ('juicebox_pro_1.5.0/web/jbcore').

If your gallery is a Juicebox-Pro gallery and you use a Flickr account as an image source, you may need to increase the flickrImageCount from its default value of 50 to its maximum value of 500 on JuiceboxBuilder-Pro's 'Images' tab. (Incidentally, this maximum value is imposed by Flickr rather than Juicebox-Pro.)

Also, be sure to clear your browser's cache before reloading your gallery's page to ensure that your browser is using the latest versions of your gallery files from your web server (instead of perhaps using older cached ones).

I hope this helps.
However, if you continue to experience difficulties, please email me back with the URL to your gallery's web page so that I can take a look at the problem for myself and hopefully help further.
Thank you.

If you need to display your gallery on demand (when a button is clicked), then I would recommend one of the following.

(1) To display the gallery on a page of its own, use a regular HTML link (an <a> tag) to open the gallery's 'index.html' page. You can incorporate a Back Button in your gallery to allow users to return to the original web page.

or...

(2) To display the gallery within your web page alongside other content, wrap the gallery's embedding code inside a JavaScript function and call the function when required.
To see this in action, create a sample gallery with JuiceboxBuilder-Pro and use the following code as the gallery's 'index.html' page.
Load the 'index.html' page in your browser and you'll see that the gallery is displayed when the 'Load Gallery' button is clicked.

<!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>
        <title>Test</title>
    </head>
    <body>
        <div id="link">
            <input id="load" type="button" value="Load Gallery" />
        </div>
        <script type="text/javascript" src="jbcore/juicebox.js"></script>
        <script type="text/javascript">
            function loadGallery() {
                new juicebox({
                    containerId: "juicebox-container",
                    galleryHeight: "400",
                    galleryWidth: "600"
                });
            }
            $(document).ready(function() {
                $('#load').click(function() {
                    loadGallery();
                });
            });
        </script>
        <div id="juicebox-container"></div>
    </body>
</html>

Sorry, my mistake. Try searching for 'juicebox_options' with an underscore instead of a dash.
Apologies for the typo. I've corrected my post above.

Thank you for posting your query with additional information over in the Showkase forum.
https://showkase.net/forum/viewtopic.php?id=344

I'm not sure what you mean.
Can you explain in a bit more detail what information you're looking for? Thanks.

If you're looking to install a pro viewer into Showkase (across all themes), instructions are here.

I think your problem is due to your site having 3 levels in its navigation menu (gallery index pages grouped under other gallery index pages). Showkase natively supports only 2 levels in its navigation menu and it is not possible to group a gallery index page under another gallery index page in the interface. A gallery index page can be only a top-level menu item.

Normally, when viewing a top-level page, the navigation menu displays all top-level links and also links to pages grouped under the current page. When you select a second-level page, the navigation menu remains the same.
Your problem seems to occur when you select a third-level page (a gallery grouped under a gallery index page grouped under another gallery index page). Everything in the navigation menu displays and works as expected until a third-level page is selected when the navigation menu displays only top-level links.
I'm not sure how you achieved a third level but Showkase was not designed to support or handle this. (It sounds like you would need a third row of links in the navigation menu and this is not something that Showkase has ever been able to do.)

As far as I am aware, there were no changes between v1.6.1 and v1.7.0 with regard to the navigation menu.
Perhaps you had made some manual modifications to the v1.6.1 source files (or maybe to your theme's 'custom.css' or 'custom.js' files) which were overwritten when you upgraded to v1.7.0.

If you are sure that v1.6.1 displayed the links in a way that v1.7.0 does not, then I would not ordinarily recommend using an older version of any software but maybe reverting to v1.6.1 might be the easiest remedy for your problem (as long as your site does not suffer from any of the bugs fixed in v1.7.0).

Incidentally, we have a dedicated Showkase forum here.