When using LDAP authentication and a linked external group, I found my user's group membership being not synced and when digging deeper into I found my user created with umlaut in the dn, so it is stored base64 encoded and also the member attribute set for the group is it. This seems to be the problem because all other users are synced correctly. To make it even worse sync works in the way it removes my user from the group leaving me without permissions.