The error above is the outcome of running Udate-MailboxDatabaseCopyStatus -Identity DB_NAME -DeleteExistingFiles and the DB status went to Failed and Suspended. The reason is that the backup was kicked off and the handle for the DB was no longer available.
You could check the status of the backup on the DB by running Get-MailboxDatabaseCopyStatus -Identity DB_NAME | fl *backup*
Two ways to get over this, either to wait for the backup to finish or to reboot the server and stop the backup and that should fix the issue, the seeding process needs to start from scratch.
#1 by AneeshS on 18 March, 2018 - 8:25 pm
The issue could also come up in case you have tried to rename the Database (scenarios where Database back in not in question). Refer Blog – https://social.technet.microsoft.com/Forums/office/en-US/b96c78f0-a0b3-4656-be21-80eb3d8001ac/unable-to-add-exchange-mailbox-database-passive-copy?forum=exchange2010&prof=required