Cool, glad you figured it out. Seems like a nice easy fix for you.
And I'm glad you didn't find the 1GB+ thing I was hinting at. A while back, I had the same problem as you, unable to publish to Steam, and in my case it was because I had somehow inadvertently re-entered the entire main campaign inside my mod folder, resulting in a *massive* .pak file size that Steam didn't want anything to do with.
