As soon as I enter the room where he's hiding in, all NPC attack me and my companions (probably because I killed every paladin in the dungeon, but it's not like I had a choice). He still talks to me though, but once we're finished he becomes an enemy as well. I'm guessing he's a pretty important NPC that will be needed later on, and there's another one in the room that I can't talk to for the same reason. What the fuck am I supposed do to now ?

I managed to fix it. Should it happen to anyone else, here's what I did : before entering the room I fired the soldier (the one from Hillbrandt) from the team, gave the order to hold weapons as I couldn't fire my hound... then talked with Christoffer and immediately talked to him again after the conversation was over. He, along with all the other paladins in the room, was no longer attacking me. They were still after the dog though, and killed him, but after I ressurected him everything was fine.

I tried once but it didn't work...

Strange, worked for me. And I killed some Masters beforehand ( ahh Instant Death, you make killing everything so sweet )

I had the same problem as a Priest !
Now I am playing as a Paladin

I ran into this problem as well, playing a priest.

The solution to disband party members didn't work for me because there's also a bug where Jacques de Pulain will not rejoin you after asking to disband. You can ask him to, but nothing happens when you click that dialogue option.

So what I did was I killed all of my party members with spells on floor 3 right before I went down to meet with Christoffer, then I went down to level 4 and spoke to him without fighting anyone. After which everyone still attacked me even though they were green.. so I then quickly ran to the portal at the top of the room to escape. When I returned to the room (back through the portal again), they were all friendly and did not attack me.

Went back up the stairs to cast raise dead on my 3 party members and they no longer had any hostility problems either from that point on (obviously I didn't get into any more fights on floor 3, as that might mess things up again).

There have been two attempts to fix this issue - in patches 1.10.14 and 1.10.17. But if the issue still persists, I'm afraid it won't be fixed anymore because the problem is somewhere deeper (too much deep to be fixed now).

The issue of Jacques de Pulein not rejoining the party was fixed in 1.10.18 (and the fix will become active once this patch will be released - no ETA on that though).