From JC2-MP Documentation
This page lists problems that people have encountered along with known solutions. Only issues that relate to the current Steam version will be added. The old troubleshooting page can be found here.
If you have problems launching the game or otherwise find the mod unplayable, please post in the troubleshooting forum or ask for help from a developer or moderator on our IRC channel. Solutions that have been known to work will be added here.