Unfortunately, as you have discovered, the current version of WP-Juicebox (v1.5.1) is not compatible with the new Gutenberg editor within WordPress 5.0 (and later).

We hope to release a version of WP-Juicebox which is compatible with WordPress 5.0 very soon.
In the meantime, there are two things that you can do.

(1) Install and use the Classic Editor plugin (written and maintained by the WordPress team) from here.
This replaces the Gutenberg editor in WordPress 5.0 with the familiar TinyMCE editor and, in doing so, restores WP-Juicebox functionality.

... or:

(2) Try the new version of WP-Juicebox which you can download from here.
It has not yet been released but it has been designed to work with the Gutenberg editor in WordPress 5.0.
The new plugin adds an 'Add Juicebox Gallery' button to the Gutenberg toolbar which, when clicked, displays the regular WP-Juicebox gallery settings window.
After clicking the 'Add Gallery' button, the WP-Juicebox gallery shortcode is entered into the page or post as a Gutenberg 'Shortcode' block.
Here's how to then add images to your WP-Juicebox gallery (essentially attaching images to the page or post) from within Gutenberg:

• Add a Gutenberg 'Gallery' block to the page or post containing the WP-Juicebox gallery shortcode.

• Upload images from your local machine by clicking the 'Media Library' button (in the 'Gallery' block) and dragging and dropping the images into the media window.

• Wait until the images have finished uploading.

• Reorder the images (if necessary) by going to the 'Create Gallery -> Media Library' section, selecting 'Uploaded to this post' and then dragging and dropping the images into the order you require.

• Close the media window using the cross at the top right (do not click 'Create a new gallery').

The new version of the plugin also adds the ability to create a new gallery on the 'WP-Juicebox -> Manage Galleries' page.
This is simply an alternative way to create a new gallery when using an editor which does not support the 'Add Juicebox Gallery' button (such as CodeMirror).
After creating a gallery via this method, you would need to copy and paste the shortcode into the page or post (and then attach images to the page or post as usual).

If you are a Juicebox-Pro user, please remember to upgrade the plugin to Juicebox-Pro (as it comes bundled with Juicebox-Lite) by following the instructions here.

Please remember that the plugin has not yet been released and is still officially in an experimental state (although I have tested it myself and it should work fine).
If you choose to try the new plugin, please let me know how you get on and if I can be of any further assistance.

What you are describing is the Splash Page.

On mobile (small screen) devices, if the gallery is embedded in a web page alongside other content, then Juicebox displays the Splash Page (an image placeholder for the gallery) which, when tapped, expands the gallery to fill the browser viewport (to give the main images as much room to be displayed as possible).
In Small Screen Mode, after tapping the Splash Page (or if the gallery is presented on a web page of its own and the Splash Page is not used), the user will be presented with a grid of thumbnails from which a main image can be selected.
When a thumbnail is tapped, the thumbnail page is replaced with the chosen image. (In Small Screen Mode, the thumbnails and main images are displayed on different pages to maximize the area available for the images on small screen devices.)
The user can return to the thumbnail page by tapping the 'Thumbnail' button on the gallery's Button Bar (and can return to the original embedding page by tapping the 'Close Gallery' button).
Please see the Screen Modes support section in the Gallery Tour for more information.

Yes, it is not a matter of changing the gallery's size but rather setting specific configuration options.

You can choose to not use the Splash Page by setting showSplashPage="NEVER" (in  JuiceboxBuilder-Pro's 'Customize -> Splash Page' section). Using this setting, the thumbnail page would initially be displayed instead of the Splash Page on small screen devices. If you want to skip the thumbnail page and go straight to the first main image in the gallery, then set showSmallThumbsOnLoad="FALSE" ('Customize -> Thumbnails').

Alternatively, you could force the gallery to be displayed in Large Screen Mode (which, by default, does not use the Splash Page and displays the thumbnails and main images together, like in desktop browsers) on all devices and in all browsers by setting screenMode="LARGE" ('Customize -> General').

If you choose to continue to use the Splash Page, then you can configure it (for example to change the image displayed) using the Splash Page configuration options listed here.

I hope this helps to clarify things and gives you a few options to try.

It looks like all your images (both landscape and portrait) are justified left due to setting imageHAlign="LEFT" in your gallery's 'config.xml' file.
Setting imageHAlign="LEFT" will ensure that the left-hand edge of each image is flush against the left-hand edge of the image area within the gallery.
Set imageHAlign="CENTER" instead (the default value) and your images will all be horizontally centered within the gallery's image area.
For reference, imageHAlign can be found in JuiceboxBuilder-Pro's 'Customize -> Main Image' section.

If you save each gallery to its own unique folder and re-save a specific gallery to its own specific folder, then no other gallery folders should be touched when re-saving the gallery.
I cannot replicate the problem you describe but it you can provide a certain folder structure and set of instructions which will allow me to reproduce the problem, I'll certainly be happy to investigate further.
Thank you.

When viewed on a mobile device, Juicebox automatically switches to Small Screen Mode by default to display the gallery (where thumbnails and main images are displayed on separate pages to give the main images more room to be displayed).
Please see the Screen Modes support section for more information on how Juicebox adapts to different devices and screen sizes.

If you want to force your gallery to always be displayed in Large Screen Mode on all devices and in all browsers, then set screenMode="LARGE" in JuiceboxBuilder-Pro's 'Customize -> General' section.

I hope this helps.

Unfortunately, as you have discovered (and as noted in the 'Known Issues' section of the JuiceboxBuilder User Guide), JuiceboxBuilder does not accept images with apostrophes in their filenames.

The apostrophe (single quote) is a reserved character and, as far as I am aware, there are no immediate plans to allow JuiceboxBuilder to accept this character.

As an image filename forms part of a URL when uploaded to a web server, it would be wise to use only web-safe characters within image filenames. Please see section 2.3 of this document for details.

You should have no trouble (either in JuiceboxBuilder-Pro or on your web server) if your image filenames use only these web-safe characters.

With this in mind, I would recommend renaming your images and their corresponding imageURL entries in your gallery XML files.
You can batch rename multiple images very quickly with a third-party tool such as Bulk Rename Utility.
You can also quickly and easily rename all imageURL entries in an XML file (or in multiple XML files) at once with a single global search and replace action in a text editor which accepts multiple files and supports regular expressions such as Notepad++.

I realise that this will add a couple of steps to your workflow but, as JuiceboxBuilder does not accept filenames with apostrophes, renaming your images (and their corresponding imageURL entries) is the only way for JuiceboxBuilder to convert your SimpleViewer galleries (and with the right tools, renaming the files and imageURL entries should not take too long).

Just to let you know, modifying the source code is not supported so I hope you understand why I could not recommend or suggest such modifications.
Please see this FAQ:

That said, I hope you are able to achieve what you are looking to do.

@petbau

I have found your Juicebox-Pro (Single Site License) purchase in our transaction records and have sent you a new download link (to the email address you used to register here in the forum).

If you ever need a new download link in the future, just fill in the Juicebox-Pro Download Link Request Form and we'll send you a new one.
Thanks!

As far as I am aware, there are currently no plans to support videos but please post suggestions for future versions in the Feature Requests forum thread where they will be seen by the developers. Thanks!

Thank you for posting the update to your problem.
Hopefully you'll get on OK with the latest version (v1.5.1).

You can upgrade an online gallery by replacing the gallery's existing 'jbcore' folder with the 'jbcore' folder from the Juicebox-Pro v1.5.1 zip package ('juicebox_pro_1.5.1/web/jbcore').
Full instructions for upgrading JuiceboxBuilder and existing galleries can be found on the Upgrading Juicebox support page.

If you encounter any further problems, please post back and I'll investigate further.
Thank you.

I'm not sure if you are referring to navigation links within the Showkase menu (on all pages including Juicebox gallery pages but not actually within Juicebox galleries themselves) or navigation links within Juicebox galleries (perhaps embedded within image captions).

Juicebox has its own color configuration options (in each gallery page) which are independent of the Showkase color settings (in 'Site -> Customize Theme').
The text color for all Juicebox gallery text can be found in the 'Lite Viewer Options' section of each gallery page.
(You can set default values for all subsequently generated galleries in 'Site -> Customize Viewers' if you like.)

If you change any Showkase colors (in 'Site -> Customize Theme -> Color Palette'), be sure to set 'Site -> Customize Theme -> Color Scheme' to 'custom' to use the custom color palette settings.

I hope these notes help,
However, if you continue to experience difficulties, please post back with the URL to the web page in question and let me know exactly what links you are referring to and I should hopefully be able to let you know how to change their color.

If you encounter the "Config file not found." message whilst trying to view a gallery locally (from your computer rather than from your web server) 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 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 this does not describe your problem, then more general information on the "Config file not found." message can be found in this FAQ:
When I view my gallery I see the message 'Config file not found'. How do I fix this?

I hope my notes above help.
Please let me know how you get on and if I can be of any further assistance.
If you encounter the "Config file not found." message in an online gallery, then please post the URL to your gallery's web page so that I can see the problem for myself and hopefully help further.
Thank you.