This is a wonderfully frustrating error. Veeam have a few options for fixes, including registry modifications and updating to the latest patch releases. This is generally caused by the temporary VSS service (in this instance) used by Veeam to capture application aware processing.
Well, if none of this works, go back to the crazy option of:
Editing the job and un-ticking the Enable application aware image processing option, which is under the Guest processing section.
Retry the failed job. This should complete the failed VM’s successfully.
Edit the job again and re-tick the above option.