my uploaded pictures have been deleted all at once for the second time within one week in an active project.
Just to be sure: You have uploaded the images via the "images and media files" feature, is that correct? Or are we talking about images uploaded by respondents in the questionnaire?
All in all, they take up about 20MB. Could this be the problem?
Probably not. You have 64 MB storage for every survey on www.soscisurvey.de, but...
On the other hand, sosci survey is run with our private university server.
... we have no control over what is happening on a private server. If there is a script that deletes files, SoSci Survey cannot do anything about that. As we never had such problems on our survey servers, it is very likely that the deletion is not triggered by SoSci Survey, but something else on the server.
Please speak to the person who is responsible for that server. Maybe they had a loss of data, and restored something form the backups? Maybe they have a script that misidentified you images as malware? Maybe the server moved, and they forgot files?