After reconfiguring VZshadow in a particular VM as per Step 5 in SOL93858: https://support.quest.com/SolutionDetail.aspx?id=SOL93858, backup job for that VM fails with "API Call failed with message: A general system error occurred: Protocol error from VMX"
The same issue is seen in Virtual Center Tasks and Events. The root cause of this issue is unknown.
Recreate the backup job for the VM that you reconfigured VZshadow with. Backups of this VM should now complate successfully.