Thank your for your great and quick support!
I think I found a hack to force eGroupware to always use DATE-TIME with correct value format.
At least it did work for me synchronizing calendar, address book and tasks (infolog in eGroupware) .
When it proofs to be stable, I will send it to the eGroupware community site - maybe they are interested …
Btw. we are not so far apart, I live in Mödling 😉
Bye, Peter 2.0