Path already mapped in workspace error with CCNet and TFS

Had a problem with CCNet that kept me here till midnight where try as I might, I just couldn't get a build to not fail with the dreaded "Path ... is already mapped in workspace ..." error:

Microsoft.TeamFoundation.VersionControl.Client.MappingConflictException: The path C:\Builds\etc\Working is already mapped in workspace someworkspace

We use a different workspace for every CCNet project to avoid collissions, and to maintain uniqueness we keep the workspace name the same as the CCNet project name. I couldn't find the workspace in question, and was pretty sure I'd already deleted it. In fact I'd used TF Sidekicks to delete all the build user's workspaces, and it still didn't work. So what was up?

Fortunately in a post 'How to handle "The path X is already mapped in workspace Y"' I learnt of the mappings cache file on the client PC, in the user's Local Settings\Application Data\Microsoft\Team Foundation\1.0\Cache\VersionControl.config file. Just nuking workspaces on the server isn't enough!

So to be sure I blew away the build server's local profile entirely, and that finally fixed it.