Hi julio,
Thanks again, but after spending my afternoon on this issue, I gave up and restored the backup from yesterday...
I can assure you, nothing got updated. The ui showed the same version numbers and still wanted to update the modules. What is more important, the Domain controller and filesharing service was down constantly.
I just checked again the updates and the version numbers in the latest version column have been changed. Now they are lower then they were in the afternoon. Check the screenshot in my first post and compare with this one. I'm sorry about saying that, but this is kind of ridiculous and shameful.
BTW, I still cannot update the core module, however, the system still works. That's already something right? I didn't even try to update the other modules. I think, that would be a best practice for zentyal, at least for the development edition.