A number of factors can cause this. To resolve I would select the VM via the vmPRO GUI and then edit it by turning Export and CBT (if enabled) off.
Then select the VM again and turn Export and CBT back on. This will cause vmPRO to re-discover the VM which will put the VM back in /export.
Run a test backup to see if the VM backs up with no issues. If you get the same error then generate a support bundle and upload it to this discussion so I can take a look at it.