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Cannot open mailbox System Attendant

Today, Get-MigrationEndpoint error on local Exchange gave this error:

"Cannot open mailbox /o=XXXX/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Configuration/cn=Servers/cn=XXXXXX/cn=Microsoft System Attendant.

Standard checking for all arbitration mailboxes
get-mailbox -Arbitration

All mailboxes are in place. So this must be something else...

So, let's check database where this mailbox exists:
get-mailbox -Arbitration migration* |select database


Then, check database status:
Get-MailboxDatabaseCopyStatus databasename

OK, so now we can see that mailbox is sitting inside dismounted database.
If you mount it again, it'll fix this error.
But you can of course mount database and move the mailbox over to the desired database using:

New-MoveRequest  "Migration.8f3e7716-2011-43e4-96b1-aba62d229136" -TargetDatabase databasenamehere