For all who may have the same issue:
The reason is a change of the group names in the Managed Devices section. If you change the group name after assigning a mobility controller to that group where the mobility controller is part of a cluster, the cluster configuration of the controller cannot be changed anymore.
Changing the group name back will enable you to change the Mobility controller cluster configuration.
For this reason, I highly recommend to think about the group naming in advance.
Moreover it would be nice to have a mechanism implemented that prevents to change the group name.