Hey Scribe and branjo,

thank you for your help. As I expected my problem to be connected somehow to an earlier malfunctioning installation of BOINC, a "clean" reinstall would be a good idea. And I learned by SekeRobs suggestions, that maybe user rights can be part of the problem as well. So your both ideas are great, I should have tried that earlier. I did a new installation as administrator, installed it as screensaver, but not with restricted rights and did the installation for all users. And I have choosen new locations for both, the program and the data.

This (maybe alltogether, maybe only a part of it, but I dont know which one) was the solution, now it works!!!

Thanks to everyone contributing to the solution.