I have the same issue after setting up a DAG between my two exchange 2010 SP1 servers.
Does this link seem to be the culprit?
Understanding Backup, Restore and Disaster Recoveryhttp://technet.microsoft.com/en-us/libr ... 76874.aspx
Be aware of these limitations when using VSS for backup and restore of Exchange data:
The VSS plug-in that ships with Exchange 2010 can be used to back up volumes containing active mailbox database copies or standalone (non-replicated) mailbox databases only. It can't be used to back up volumes containing passive mailbox database copies. To back up passive mailbox database copies, you need either Microsoft System Center Data Protection Manager or a third-party Exchange-aware VSS-based application.
Passive mailbox database copies are backed up using a separate VSS writer in the Microsoft Exchange Replication service. The Microsoft Exchange Replication service VSS Writer doesn't support restores. Although you can back up a passive mailbox database copy using Microsoft System Center Data Protection Manager or a third-party Exchange-aware VSS-based application, you can't perform a VSS restore directly to a passive mailbox database copy. However, you can perform a VSS restore to an alternate location, suspend replication to the passive copy, and then copy the database and log files from the alternate location to the location of the passive database copy in the file system.
I am not sure how the mailbox is being called by veaam and exactly what Exchange writers it uses but at least for me I've narrowed down having two active/active databases on one server and not having a problem. It seem to me that it's also plausible as right now I am in the middle of moving many of our users to different mailbox stores based on location. My one mailbox store is over a 10mb link and DAG between the two databases (and the move to that server's active database) takes place over that link. I was wondering if it's also possible that simply seeding the DAG database requires the VSS writer to be in-use and this causes the issue with Veaam/Microsoft's VSS writer when another application tries to gain access to perform the backup for Veeam. What concerns me most is the fact that the passive database from my other mail server resides on the same server that at times backs up and other times does not and I am wondering if the tech net article above is the reason for this.
Usually after a few tries or possibly letting the database sit idle and running the job again later that night fixes the issue. What I do notice is when that happens my traffic between sites are lower - which in turn means the replication of DAG between them is also lower.
For the record Veeam is setup to backup the server from the same site and uses 1 gbps link to do so for the exchange server that is on the same switch.