i got it running.
Just to clarify:
master.domain.local -> UCS 4.2 (Central LDAP, installed the kopano4ucs packages (lib, udm, schema)
i looked at an ‘complete’ UCS domain, where also kopano is installed on an UCS-Server.
So i installed the same packages on my Master.
mail.domain.local -> Debian 8, here i installed the Kopano Core & Webapp, and i want these installation to get the Kopano users from my UCS Master
The main problem was, that the UCS master didn’t knew, that there is an Kopano Installation within its domain.
The point was, that the installation didn’t created the custom attributes at “local.domain:/univention/custom attributes”
Normaly these entrys should be created automaticly with join scripts, as @Grandjean said, i have no idea why this didn’t happen.
i created all these entrys manualy and now it works like a charm.