I am having the exact same issue that Nightwolfe, another user in the forums, is having.
"The files of mod 'Main' and/or its dependencies are invalid. This can be caused by corrupt files or some files may have been changed. You wont be able to play this mod in multiplayer."
I can't play multiplayer.
Moreover, when I chain my characters together they will not move as a unit. I have to manually move each character, which makes things pretty clunky.
Interestingly, when I verify integrity of game cache it says one file is corrupt or missing and downloads the file. If I do this *again* it says everything is good to go. When I start the game, I have the same issue. So then I RE-RE-verify, and it again claims one file is corrupt and redownloads.
I have tried the fixes that Raze mentioned to no avail.
Anyone have any ideas?