Have the host check the characters to see if any are encumbered. There is a known issue that can cause the client to crash in multiplayer when loading or joining if a character is encumbered (the main cause of that was fixed in one of the last updates, but a few people have still run into this). In that case, the host dropping or redistributing the excess weight and saving the game should let the other join.
Thank you, that was exactly the issue! Since we started to track our weight we haven't encountered this bug even once.
