This is happening with me too since the Update 174992 was released.
The day before the update I could play those UGC co-op mod maps with my girlfriend, after the patch we couldn’t any more due to her being kicked from the lobby after the file transfer failed.
This was the sole reason I bought 2 copies of the game…
If you don’t plan to release more Co-op campaigns that a lot of people like me would even pay for, at least please don’t break the community ones.