I solved the problem temporarily by installing an elder version of BOINC, 6.6.36. This one works without problems.
There seems to be an error handling interruptions of the online link during downloading of new workunits.
Hope this could be fixed in the nest version.
Greetings to the Community
Jochen
Greetings Joachim,
Glad you were able to find a temporary solution to your issue.
Looking at the BOINC forums you are not the only person having some issues with that 6.6.38 and I read that there may be a fix coming in a future release.
-Spark