Open the save up in Horizon and rehash/resign with that. If that doesn't work, try moving some of the contents around to a different save (Horizon can do that too). Also, as Jappi suggested, try "unmodding" your save with his tool, as that actually has a decent chance to fix it.
The Steam version of all PC games are different somehow from the Windows Live versions. I don't know how, but they handle the files differently. It has something to do with how the games are formatted so that they mesh with Steam's system.



















