1.
We were playing and my friend crashed, so i decided to wait for, as she had to restart her pc and the game. That time i still had her character in my party and she could not join, nor could i kick her out while i was still in a gamesession. Only way was to return to menu to then kick and reinvite her. That way we lost some progress in the story and were set back 1 already unlocked waypoint.

2.
I got downed and she could not click my character, so while she waited in the game i left to the main menu. There i clicked on play again and managed to join in the game, BUT, to my surprise i ended up at a waypoint just where you have to defend the cannon. While she was in the necropolis waiting. Also, when my character spawned a lot of cutscene audio was suddenly playing, the intro as well as the one from that region i spawned(more i could not figure from the audio-salad).
I then clicked on her character party-icon to teleport to her and we continued, or so we thought. The part where you need to click those big wheels on two sides to open the pathway in the middle was not possible for me to pass anymore. For her it was done and it looked as if she walked through the air while i could not pass. The wheels mentioned were not clickable for me, but since she could pass i figured i could just teleport to her once she was on the other side. It worked, but obviously is not the solution to the problem. Please look into the progress if both players did activate the contraptions, or let each player activate them for them self. Rather set a flag for progress for each player based on cumulative progress of the party.

3.
Similar to others already experienced solo, we had situations where we spawned in the town and each of us saw the other character half way down to the hips in the ground. That lasted for a bit and was set back with feet on ground after some walking.