Hello,
I started a new game and in all my wisdom I deleted the problematic save. It was indeed only 1 save that caused this issue. If it occurs again, I will send it to you.
Some info that I can add... This *seemed* to have happened when a friend tried to(joined?) my server and was playing as the Red Prince so I think it's multiplayer related. If I remember correctly we experienced the same issue back in the original DoS using the 4-coop mod that a user published at the time. It seems that the new system implemented in DoS2 is very similar to that mod in that regard since as back then, when the coop required the henchmen to be unlocked in order for the 4 characters to be available, in DoS2 if you try to login with only 1 character a "server full" message will pop up so you have to recruit 4 heroes around Fort Joy for a full 4-player coop.
I would expect a more like "drop-in" system. If someone was invited-wanted to join, he would have the chance to create a new character. Then a pop up would appear at the host notifying him for the race and class and which NPC-controlled hero he would like to replace. I understand that this might require a complete redesign of the game engine but it was just a suggestion

We ended up creating a brand new multiplayer game where all 3 created new characters at the same time. After that, we played for 7 hours straight without any lag, crash, bug or anything we can remember. Great game and really polished for an alpha version.
Keep up the exceptional work!