My friend and I are having an issue with hosting a game or checking our Mods, this is the error message whenever we try to access these.

I Uninstalled and reinstalled and that fixed it for me but if that does not work try verifying the game cache

From the other bug report:

A fix is getting built to address this right now. In the meantime you can keep trying and it'll "eventually" work. I seems to be related to Steam returning an empty list of mods back to the game due to a bad connection on Steam's end.

Changed Status to Fixed