That's the information I was looking for...
So *if* you are only backing up one volume *and* that volume is backing up/verifying successfully *and* you can restore from the backup *and* you get the un-named volume error *and* Retrospect carries on regardless -- I'd just ignore it. If the error is causing other problems, eg killing the script while there are still other machines to process, re-arrange things so the erring machine is the last to be done.
If the error is truly killing the system, eg popping a dialog that must be dismissed before continuing, I'd look into script triggers and a GUI-targetted AppleScript to automatically dismiss the dialog so RS can continue.
Some things are easier to work round than to fix 😉