My friend and I are still having a similar issue, as recently as several hours ago. He's the host, and he's using a wizard with lone wolf so he doesn't use a companion. I've got a knight with a companion. What seems to happen is one of two things:
First: Upon entering combat, he gains control of my knight and I'm stuck only able to control the companion. Not even reassigning seems to fix it, and if I try to exit the game and rejoin I just get a black background and a black UI. Basically the skill bar is blank, and the minimap is black along with the screen. In order for me to re-join his game and not have the issue he has to exit to the main menu and reload the save, then I join and things are fixed.
The second issue is that the companion is randomly assigned to him at the start of combat. From what I can remember noticing, it seems to happen more frequently if he's already in combat with something and then I join him.
So our best bet is to email support with the report file generated by the tool?
