KB1680: VSS Timeout when backing up Exchange VM
https://www.veeam.com/kb168028.09.2012 · To overcome this VSS limitation, Veeam Backup & Replication utilizes the Microsoft VSS persistent snapshots technology for backup of Microsoft Exchange VMs. If Microsoft Exchange fails to be frozen within the allowed period of time, Veeam Backup & Replication automatically fails over to the persistent snapshot mechanism.