Enable the Microsoft Exchange Replication service VSS writer. This VSS writer is enabled by default when you install DAG, however legacy backup software might need this writer to be disabled and that could be the reason why it was disabled manually.
Open Registry Editor and navigate to
HKEY_LOCAL_MACHINE\Software\Microsoft\ExchangeServer\v14\Replay\Parameters (Exchange 2010)
HKEY_LOCAL_MACHINE\Software\Microsoft\ExchangeServer\v15\Replay\Parameters (Exchange 2013)
Change DWORD value named EnableVSSWriter from 0 to 1.
Then restart the Microsoft Exchange Replication service.