Nach längerem Suchen habe ich nun folgend Eintrag gefunden:
An error occurred while trying to select database copy 'MDB1' on server 'server1' for possible activation. The following checks were run: 'IsHealthyOrDisconnected, IsCatalogStatusHealthy, CopyQueueLength, ReplayQueueLength, IsPassiveCopy, IsSeedingSource, TotalQueueLengthMaxAllowed, ManagedAvailabilityAllHealthy, ActivationEnabled, MaxActivesUnderPreferredLimit'. Error: Database 'MDB1' is already hosted on target server 'server1'.
Anschließend schwenkt er die DB auf den Server2 um.