Retrospect: "-50, feature not supported" Error

I'm using Retrospect 2.0ci on an Apple Workgroup Server (AWS) 95. I encountered the following error during an unattended backup:

Error -50 : Feature not supported

I have not made changes to my script and backups have been without error for several weeks. I cannot find a description of this error and would appreciate feedback on how it is triggered.
The "-50, feature not supported" error is reported when Retrospect tries to automatically mount a shared volume that no longer exists.

You should go to your backup scripts and remove this volume. If you are unsure which volume it is, go Immediate > Volumes, select a file-shared volume, and try to scan it. If you can scan it, it is fine; repeat the process for the other file-shared volumes.

If you get an error, you should then choose to "Forget" the volume.
Published Date: Feb 19, 2012