userClassID and userGroupClassID settings available in UserService are intended only to be used as defaults in creation of Users and User Groups.
PAPI allows custom ContentTypes to be used when creating these.

In methods loadUsersOfUserGroup(), loadSubUserGroups() and loadUserGroupsOfUser(), those settings are also used to load Users and User Groups, which will not work correctly if custom ContentType was used in creation.

These settings need to be removed from the mentioned methods. Currently no elegant way exists to load children Users/User Groups and implementation will need to load all children and check for ezuser field definition.

It also should be looked into how to do this in a more efficient way.