We recently abandoned Backup Exec and transitioned to Veeam Backup and Replication and as with most product transitions we ran into a few hiccups. Aside from having to adjust Shadow Copy space limits on some VMs and Hyper-V hosts, we also ran into snapshot errors. BE uses agents to handle quiescing while Veeam directly contacts the VM to request snapshot creation. Some of our VMs were in DMZs and other areas and not on a domain so the default credentials that Veeam was using was not able to authenticate and create snapshots.
The fix for this was to add additional credentials and map them to each of the errant VMs directly.
After we got that sorted out, the rest was a breeze.