Looking through the threads, I was amazed that the responses were “Contact Tech Support” and “Uninstall and Reinstall”. Atrocious service Western Digital.
In Vista the solution is:
Go to Users --> ~username~ -> AppData -> Roaming -> WD -> WD Anywhere Backup -> Instances
Delete all folders inside this directory. You can then go back into Anywhere Backup and Reactivate the Backup Plan.
Other versions of Windows will have similar solutions, probably in Documents & Settings -> username -> application data…etc etc.