We see the same warnings here. In our case those groups do exist, but only as normal groups, not as dynamic groups. The query causing the warning to be emitted is the one for the dynamic groups. From our log files:
Wed Aug 30 13:37:07 2017: [ 20006] plugin: ldaptiming [00000.00] ("dc=bs,dc=linet-services,dc=de" "(&(|(&(objectClass=posixGroup)(kopanoAccount=1))(objectClass=kopano-dynamicgroup))(gidNumber=40000))" uid ), results: 1
Wed Aug 30 13:37:07 2017: [warning] Unable to retrieve parents for relation groupmember: uid attribute not found.
Group ID 40000 is
mitarbeiter, and that one is known to Kopano, just as a regular group:
[0 mbunkus@merrimack ~] sudo kopano-admin --type group --details mitarbeiter