If your version of Veeam Backup & Replication is 9.5 update 2 (126.96.36.1998) or prior, there was a known issue that could be addressed by a fix applicable only to that version. However, that known issue has been addressed in update 3 and newer (188.8.131.526) so if your installation of Veeam Backup & Replication is not fully updated, it is strongly recommended that you consider doing so, or contact Support if you are unable to update and need the fix.
There is a distinct known cause for this error message when there are underlying issues with the SQL VSS writer on the guest VM. You should be able to see error messages in the guest VM’s Event Logs stating, “A critical component required by the SQL writer is not registered” or the VSS writer rejecting the event.
Running the “vssadmin list writers” command might also show the SqlServerWriter in a “Non-retryable error” state.