If you have reinstalled or upgraded the Retrospect Client on a Linux system, you may see the above error. Retrospect may also report something like "RefStation::CalculateSpeed failed; error -562, network connection reset by peer" when using the "refresh" option for the client from the backup server.
When this happens, you will want to remove the Linux client from Configure>Clients (or Sources on the Mac). After removing the client, try to re-add the client back into Retrospect.