The ACL was enabled on the uplink interface and could not be removed.
In finally was able to remove the ACL. Those were the steps necessary to remove:
- Enable "PEF" Feature in MM
- Create session ACL "remove-me"
- Log onto MD1, enter disaster recovery mode
- apply ACL "remove-me" to interface, write memory
- remove ACL "remove-me" from interface, write memory
- Disable "PEF" in MM
After this procedure I was able to form a L2-Cluster that seems ok, however the APs are not broadcasting any of the configured SSID. APs are shown "down" in Mobility Master.
I deleted the cluster again and everything is fine with just one controller.