Same as @krindi in his post in the following article, he ha a good description (and same way of problem solving) of it:
as well as
I think too that it has something to do with a silent join, maybe one that gets cancelled in the wrong way before even notifying the host/other clients in txt.chat, which blocks one bot place as being used by a person who is in the process of joining, which therefore won’t let the game finish. One friend tried to join as a fourth, but said place was blocked. Could maybe be prevented by an automatic join cancel, after a set time? I don’t know, 3 min p.e. should be enough for even the potatoest of potatos to join in. Or maybe a timer for the different stages of joining?
Too say, it happened to me the first time in 1500h of Vermintide 2, so extremely rare.