I found the problem with the growing / … the samba-log. For a other issue:
https://help.univention.com/t/nach-update-altere-clients-kein-zugriff-auf-share/9514/7
I set a higher debug-level and forgot to disable it again 
The master still has a very high cpu-load (100%) when I look at the UVMM but not in UMC on the master 
the problem with the upgrade persists