Thank you for the additional information.
I can find very little information about this theme on the internet other than this forum post.
I cannot confirm whether the information in the post is correct but it would appear that the theme is known to conflict with at least one other plugin, includes about 30 JavaScript files and allegedly contains 'a lot of JavaScript errors'. If any of this is true, then it may be very hard to debug and the only solution may be to switch themes whilst creating your WP-Juicebox galleries.
However, the WP-Juicebox 'Add Gallery' pop-up window is essentially a self-contained web page and should not conflict with any JavaScript files loaded in the standard admin section pages.

I have emailed you with an address where you can send me login details to your WordPress Dashboard.
I will gladly take a look but please be aware that I cannot be sure that I will be able to either diagnose or fix the problem.

Unfortunately, the theme you use is not free so I am unable to try this for myself.
Is the error message displayed on-screen when you initially try to view a gallery (or at some other point, such as trying to go fullscreen)?
Are you able to try on a different web server to see if this makes any difference?

@jhob

Introducing a cross fade transition is on the roadmap and should hopefully be included in a future version of Juicebox-Pro (although I do not know when this may be).

This embedding code will display one image (via CSS) as a background to the gallery when the thumbnails are displayed and a different image when the thumbnails are hidden. In order for the background to be visible, the gallery's own background must be transparent.

<!--START JUICEBOX EMBED-->
<script src="jbcore/juicebox.js"></script>
<script>
    jb = new juicebox({
        containerId: 'juicebox-container',
        backgroundColor: 'rgba(0,0,0,0)'
    });
    
    jb.onShowThumbs = function(isShowing) {
        var element = document.getElementById('juicebox-container');
        var image;
        if (isShowing) {
            image = 'image1.jpg';
        } else {
            image = 'image2.jpg';
        }
        element.style.backgroundImage="url('" + image + "')";
    };
</script>
<div id="juicebox-container" style="background-image: url('image1.jpg');"></div>
<!--END JUICEBOX EMBED-->

On the 'Manage Galleries' page, WP-Juicebox lists all XML files that it finds in the 'wp-content/uploads/juicebox/' folder. If there are more galleries than will fit in your browser window, you shjould be able to scroll down to see the rest.
Take a look in the 'wp-content/uploads/juicebox/' folder to check that there are, indeed, 274 XML files.
Also, if you have manually edited any of the XML files, make sure that when you resave them, you use a lowercase '.xml' file extension.

You could perhaps use the Juicebox-Pro API, specifically the onShowThumbs() event, to change the background of your gallery depending on whether or not the thumbnails are displayed.
The online demo notes in the 'API Output' text area when the thumbnails are displayed or hidden. You could view the source of the page in your browser and modify it to suit your own needs. Please note that knowledge of JavaScript will be required.

You could perhaps try adding CSS such as the following to the end of the 'jbcore/classic/theme.css' file (though please note that this is untested and I do not know if it will work or will have any unwanted knock-on effects).

.jb-splash-holder img {
    left: 0px !important;
    top: 0px !important;
}

The text is currently hardcoded into the 'juicebox.js' file (which is packed and obfuscated and cannot be modified).

However, the ability for a user to enter custom text for the on-screen tooltip text and messages (similar to the SimpleViewer languageList configuration option) is on the roadmap and will hopefully be introduced in a future version of Juicebox-Pro (although I do not know when this may be).

If you are trying to get your gallery to work locally on your computer, try using paths such as:

baseUrl : 'file:///C:/Users/.../Sites/drupal/sites/all/themes/mg/galleries/Profile_Gallery/',

However, this will not work when uploaded to a web server (as the paths refer to locations on your own computer's hard drive to which internet users have no access).
You would need to swap the paths for relative URLs or absolute URLs such as:

baseUrl : 'http://www.example.com/Sites/drupal/sites/all/themes/mg/galleries/Profile_Gallery/',

No. Juicebox-Pro displays only one main image at any one time.

If you are referring to the Juicebox skin for jAlbum, then the menu can be removed (which will result in a single gallery of images in the root input directory) by going to 'Album -> Settings -> Advanced -> General' (from the drop-down menu at the top of the jAlbum window) and deselecting the 'Process subdirectories' checkbox before clicking 'OK' and making your album.

With Juicebox-Lite, all thumbnails are 85 x 85. With Juicebox-Pro, you can set the dimensions of the thumbnails to whatever you like using the Juicebox-Pro Thumbnail Options thumbWidth and thumbHeight.
You would first need to upgrade the skin from Juicebox-Lite (which it comes bundled with) to Juicebox-Pro by following the instructions on the skin's web page. You would then need to tell jAlbum what size of thumbnail images you would like it to create ('Album -> Settings -> Images -> Image bounds') and enter the corresponding Juicebox-Pro configuration options into the skin settings 'Pro Options' text area as follows:

thumbWidth=180
thumbHeight=90