Thanks for the help.
Saved the vCard, found there 2 records of groups.
It’s not clear how they signed up, but after their removal, the synchronization went with the groups.
SNI stoped working after update
My server delivers the right cert when checking with chrome or the ssl-check-service you recommend. It also worked in previous versions of davdroid. With the new http lib it uses the default host cert and fails.
Duplicate of #205
I am runnung 4.4.2 with the moat recent davdroid version from googles appstore.
There is already a bugfix release 0.5.10.1 that fixes the sni issue. Wait some hours and you will receive the update automatically.