Well, if the mod didn't change, and was working fine before, and the forum didn't change, and was working fine before, and the server didn't change, but was working fine before....what changed?
Something has changed either in the codebase or on the server. Now we need to find out why that is because the problem you are having is likely the same reason as the others with the same error.
What happens with a fresh install of S-O-M? I'd like to know if the S-O-M code is being changed or if there is some other problem like a security hole in vB responsible.
Make sure to vies the readme for uninstall instructions before a re-install.