Hi,
I got always issues with Coop games, now I found out why.
When a new version of a map is released, normaly you can download the new map files and play the game. The map files will be placed each time in a new folder for a new version. But sometimes it does not download all Files neccecary. I could solve the problem by copy pasting map files from an older version into a never one, by not overwriting the changed files.
Perhaps have a look on the download Manager in FaF.
Greetings Blacky