I posted the problem to EGroupware as well, but did not get an answer there.
I finally chose to swith to another CalDAV-Sync app which seems to work better with EGroupware.
so in version 1.4 everything was fine. I now have version 22.214.171.124-ose. I use DAViCal with a debian.
My old account’s still work(sync), but when I add a new one it fails recognizing the calendars.
Thunderbird Calendar-Plugin still works and is able to subscribe calendars from DAViCal (old and new)
Ahh and Apache2 listens on 80, 8080, and 443
Thanks for your time
403 Forbidden looks like a server problem …