I finally managed to find my way to Sixus, but as soon as I step into his room, he kills himself. According to multiple sources, he should ask me who sent me when I open the door, but this doesn't happen - instead, I just get a message about him committing suicide. How do I avoid that?

As far as i remember

I.You need to talk to Bevan in Almaria and get his quest&Info about Sixus

II. Inside the Halls...don't remove his forcefield and don't kill his Golems or apprentices, use "stealth" instead by using the backdoor (somewhere next to the lizard lairs was a weak wall that could be destroyed->leading to some caves, one of them was a secret path to sixus)

If you break through the maindoor and/or killed his supporters and/or are not send by Bevan he will commit cuicide.

Good luck, hope you have a save before you entered the Halls.