I figured it out. It seems that my backup software moved the images at a certain age into a different folder in the upload directory, thus they were not found.
Restoring a previous version (using a different backup software) solved the issue. Images are back and indeed, far more than 100 galleries are supported. My bad.
Unfortunately not.
I have 115 galleries on 110 pages (each page has one, max 2 galleries).
Now I see that the first couple of pages (9 or 10) do not load their respective gallery (e.g. first page gallery 1) anymore. The progressbar (dark gray above the images) completes but the gallery stays empty.
I noticed however (maybe a coincidence) that I created a gallery 101 and deleted it afterwards and it continued creating galleries with ID 102 onwards. Not sure if this causes the problem?
