There are a lot of things to try. Backup Exec is calling a quiescing snapshot from vSphere. You can try a snapshot manually. with the checkbox enabled for quiescing.
It could be two competing VSS snapshots on the VM after upgrading the VMware tools:
Backup Exec also has retry logic if you cannot work around this issue:
Or edit the job and deselect Use VSS on the virtual page. Just remember that you might not get a file consistent backup of the VM with this turned off.
Run the repair of the VMware tools and make sure that VSS is not selected.
Also if manual snaps are working then try the backup again. Backup Exec uses the exact same snapshot as the UI (VDDK ). Quiescing means that the machine was too busy to create a successful snap.