Here is a typical scenario of a DAG backup failure from Symantec.

If you come across an email alert from the Backup administrator like below,

Not to be panic…click on the attachment and see what the error is..and it should be like below

Then go to details…

Exchange server 2010 DAG backup failed!!! ohhhhhh.. Try to ping the DAG IP address. If no response, Logon to Mailbox server, and open Failover cluster manager.

Remember, DAG’s basics is on clustering. Click on Cluster events

Goto to the Services -> click on Cluster Services and then click Restart.

This will happen when the mailbox server faces “Dirty Shutdown” or Network connection lost.