CruiseControl.net and SourceSafe periodic error

I'm using CruiseControl.net with Visual SourceSafe and every now and again the build fails with this error:

ThoughtWorks.CruiseControl.Core.CruiseControlException: Source control operation failed: No VSS database (srcsafe.ini) found.  Use the SSDIR environment variable or run netsetup.

The documentation says this is due to running the CC.net service as Local System which doesn't have permission to connect to other machines. I am开发者_如何学运维 running the service under that user, but the build works 19 times out of 20 so I don't think it's a lack of permissions.

Has anyone else experienced this?

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This seems to be related to builds running while I'm not logged into the machine.