If you encounter a problem with mouse and/or keyboards not responding in-game but they work on either the menu screens or the Steam overlay, please try the troubleshooting steps below:
1) Disable Steam Cloud.
2) Delete all files in Steam\userdata\SteamID\AppID.
3) Start the game to create fresh (and thus different) new files.
4) Exit the game and Steam.
5) Delete remotecache.vdf in Steam\userdata\SteamID\AppID.
6) Start Steam.