The problem with your '2018nordart' gallery is with the images on your web server.
According to your gallery's 'config.xml' file (https://www.haselau.eu/jb/2018nordart/config.xml), the first image in your gallery should be located here: https://www.haselau.eu/jb/2018nordart/i … 202018.jpg
However, going directly to the image's URL in a web browser (bypassing Juicebox) displays a blank page instead of the image.
Going directly to the image's URL in a web browser should display the image (completely independent of Juicebox) but this does not happen so the problem is with the image on your web server rather than with Juicebox itself.
First of all, please double-check that the images have been successfully uploaded to your web server.
It looks like the blank page may actually be an error 404 (file not found) page so the images may not have been uploaded to the correct location on your web server.
Please also check out this FAQ:
My images show locally, but not when I upload them to my website. Why?
If this does not help, then the problem could be due to your image filenames containing special characters (such as "ü").
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.
Characters that are allowed in a URI but do not have a reserved purpose are called unreserved. These include uppercase and lowercase letters, decimal digits, hyphen, period, underscore, and tilde.
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, try renaming your images to use only web-safe characters within their filenames to see if this helps.