The Kolab GUI has some restrictions that LAM does not have. Please pay attention to the following restrictions:
Common name in LAM
The common name must have the format "<first name> <last name>". You can leave the field empty in LAM and it will automatically fill in the correct value.
Changing first/last name in Kolab GUI
Do not change the first/last name of your users in the Kolab GUI! The GUI will change the common name which leads to an LDAP object class violation. This is caused by a bug in the Kolab GUI.