Please do not just leave a one line response for issues and bugs. That helps nobody.

I need to know what happened, if the issue(s) is/are occurring on singleplayer, non-dedicated(this is the "host" option with singleplayer) or dedicated(local - on your LAN/network, or hosted - by a company) server games, when it happened and what was done/happening when it happened.

Steam has been increasingly corrupting the download and install of mods and for whatever reason.