I found a workaround in my situation for the loading crash of the second player in the lower city.

2 players :
- 1st player (host) load the game, and move the main character of the 2nd player (crown) in the camp as he lets the second character of the 2nd player (no crown) in the lower city;
- 2nd player load ;
- 2nd player moves his/her main character (crown) in the lower city (exit the camp).

Worked 5 times in a row since I made the change, and it's simple enough to be tolerable.