Exchange 2013 : Connecting disconnected archive mailbox

According to Microsoft, connecting archive mailbox after disconnection, should be easy as use connect-mailbox -archive command - similar as to use with regular mailboxes
https://technet.microsoft.com/en-us/library/ee633490(v=exchg.141).aspx
But this will not work with Exchange 2013
Instead, yo need to use:

Enable-Mailbox -ArchiveGuid MailboxGuidHere -ArchiveDatabase DatabaseNameHere -Identity MailboxNameHere

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

Database
--------
Databasename

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