My Kopano server logs the following message every few minutes:
Tue Jun 27 19:16:38 2017: [warning] Previous message logged 100 times
Tue Jun 27 19:16:38 2017: [warning] Unable to retrieve parents for relation groupmember: uid attribute not found.
For now everything seems to work fine but I think this behavior is not normal.
Using UCS 4.2-1 errata52, Kopano Core 18.104.22.168