There was a change in RBAC. Group managers are not able to add or remove members of a distribution Group even if it seems so in the Exchange Management Console.
The only options in ECP was to give them the additional permission to great and remove groups. You can create a new role that will enable this permission again. Thanks to Matthew Byrd at Microsoft who has created a Powershell script that does this for us.
Had problem with som Lync Phones (Polycom CX600). That suddenly stop signing in. Did a reset to factory deafult and the issues remained. Solution was to login with extension and pin and then connect the phone to PC using USB and login. Now I was able to login the phone using Lync client. This procedure was repeated on several phones. It seems to me that a factory default does not completely remove all old settings.