Loadup the map in solo then try to host. if dont work change host it will loadup, is a common issue, if you get an error all players should restart steam and try again. you should do this each time you want to change a map. When it work all players should see the load screen flicking 1 time.

How did you manage to get it to work with friends? Tried doing the same but it kept kicking my friend out with an error message.