Vitaly, thank you for your reply.
I just got off the phone with Veeam Support.
We established a remote session and it took the engineer about 1 minute to solve the issue
For anyone else running into a similar issue after upgrading to Veeam 9, go to Backup Infrastructure --> Managed Servers --> Standalone Hosts --> right-click on the host and select Manage Volumes.
In this case, 3 of the 4 connected volumes had a value of '0' Max concurrent snapshots. The 4th had a value of '4'.
After setting the value to '4' on all of them and restarting the backup jobs, it all seems to work again.
A big thank you to the Veeam support crew for helping me fix this.
Case is now marked as solved and closed.