I refer to the problem I already posted on the mailing list (see URL). Beside the problem described in that mail, the following additional errors occurred:
SCHULE: <some number here>
Error looking up domain users
<only BUILTIN groups are shown>
wbinfo -g --domain=SCHULE
Error looking up domain groups
By now I have updated to version 3.0.32, did not change any configuration and everything works.