?
Well, the only code path that will yield that error is if System.IO reports that the file requested is not on disk. The most likely cause of that is that the file is not actually there at the time (obviously), but that might be because anti-virus quartanteed it, a file server didn't replicate it, or something?
So, I would try to find what might be causing that file not to be there. The only way that BuildMaster will delete a file (but not the record) is a retention policy.