I've had that error a couple times.
The last time I was running stock so OC wasn't the problem.
I didn't really know what to do so I just deleted my work folder and queue and got a new WU.
Have since found a few threads which may help, but haven't had the problem again... yet...
www.fold4life.com • View topic - How do I use qfix? [How to] Submit partial result with qfix(SMP clients) - Bjorn3d.com Forums
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Server reports digital signature does not match
Some of the newer servers don't seem to like the older versions of the client.
Upgrade to the latest client. In addition to this, a corrupted queue.dat file
can cause this error to be reported.
Running qfix may help resolve this issue.
If you are running the latest client, and qfix does not rectify the issue,
report the error on the folding-community forums and delete the WU,