When backing up virtual machines that reside on a Cluster Shared Volume (CSV), Backup Exec prepares the CSV for backup to avoid data corruption. During the preparation, the controlling node is moved and pinned to the cluster node on which the backup runs. Only one virtual machine can be pinned at a time, so backups of multiple VMs on a single CSV process sequentially, one at a time.
"Snapshot provider error (0x173D): Backup is in progress. Please wait for backup completion before trying this operation again "
Backup to disk configured for more than one concurrent job, or a robotic library with two or more drives should allow concurrent backups. If two backups of different virtual machines on the same CSV are set to run at the same time, one of them will pin to the virtual node and the other will fail.
Run the VM backup jobs to the same device so that the jobs queue behind each other.
Try selecting the Hardware VSS provider if your SAN manufacturer provides one.
Microsoft VSS only supports snapping one VM at a time or serial mode backups. Some hardware VSS providers support parrallel backups of a CSV and should be unaffected by this issue.