Help, Co-op problem!

[-|WaD|- Jnr Sgt. HellsArmy]

I made my own test chamber and i want to play it with my friend, when we try to start it, the game takes us to the main menu, please help!!!

[Left4toast]

thats happend to me before

the way i fixed it is to have all my portal 2 files backed up
and reinstall portal 2