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.
I had been backing up successfully for weeks. Then I got the RPC server unavailable. I thought I tried every possible solution for this message. The first time I got it another program had grabbed one of the ports Veeam needed. This time all ports were accessible, I have the free version so the above job fix wasn’t an issue for me. Turns out it was the external usb 3.0 dock (driver?) interfering with Veeam services starting up. After I turned off the dock (as a matter of taking out the possible causes one by one, the computer came up and I started Veeam and then I turned on the dock. Everything worked! As a side note, the drive letter assigned to the dock also changed back to what it used to be (not sure if that had anything to do with it or not but I really think it was the dock driver starting before Veeam and its services.) Hope this helps someone.