volgens Zaphod Desiato.
Thanks a lot for that quick answer!
When one of the two testservers (which I mentioned before) was still running on the older version, backwards compatibility was usually never a problem. So yeah, probably the problem is something else; I can't figure out what though.
I also left out any student data simply because it wasn't needed, and the features I use in the lessons are just the basic ones: Text, mimetex formulas, images, I don't even use the "regular expressions" option in short-text questions; no plugins, nothing.
Although I'm not sure if that will make any difference, I will now try the .zip-renaming and importing the zips. Thank you!