There is an issue easy to reproduce is about the xp of quest.
When a quest is accomplish you will gain more xp per character you get for each character.

For exemple at the beginning if you go see Dallis alone without the red prince you get 200xp.
If you take the red prince both your main character and the red prince get 400xp each.
If you play with 3 friends and you get to Dallis each character get 800xp.

It is always the case for each quest : each character will get (x*nbr of character) xp.

I believe this is an issue.