Recently I encountered an issue with a client where VEAM backups were causing the DAG to fail over. This has led me to question whether or not VEAM is truly supported by Microsoft. I’ll need to scour for some Microsoft documentation to actually confirm. Either way, I digress as that is for another conversation in itself.
It seems that when VEAM is committing the snapshot the FCS sees the machine as down and in turn the DAG fails to the secondary site. In order to resolve this issue I have found that you must decrease the sensitivity of the FCS polling. This entails increasing the failure threshold as well as the frequency in which FCS polls.
You can set the maximum values via cmd prompt on the mailbox servers in question:
cluster /prop SameSubnetDelay=2000:DWORD
cluster /prop CrossSubnetDelay=4000:DWORD
cluster /prop CrossSubnetThreshold=10:DWORD
cluster /prop SameSubnetThreshold=10:DWORD
Let me know how this goes for you.