I have seen this happen when there are other servers in the protection group that are either no longer valid servers (they have been removed from the domain), or maybe just aren't available at that time. Most likely it will be option one as DPM will check
AD for all of the servers of a PG before it will modify it in any way.
I would go through the entire PG and verify that the servers still exist. If they don't, then you will need to modify the PG to remove them all at the same time. If you try to do one at a time, it will fail again as it is checking the other servers
in the group at that same time.
Thanks, Chris - [MSFT] This posting is provided "AS IS" with no warranties, and confers no rights