Edit: I changed the original because the bug I suspect I experienced was somewhat different than I first thought.
So, at specific points in the battle with the Mark II Beta Exterminator, it spawns a Unstable Exterminator Core that you have to attack if you want to do actual damage to the Mark II Beta Exterminator itself.
I went through that once, but at the second point at which it was supposed to occur, no Unstable Exterminator Core spawned anywhere to be seen, a weird Power Golem with the green swords above it spawned outside the ring behind the barrier. You can't go or shoot through the barrier. If I can't kill the Power Golem for Volatile Cores (?) and can't hit an Exterminator Core (because it's not there) the fight does not proceed.
It is also impossible to die in this fight. It is also not possible to just stand on the Exterminator and kill it over the course of an estimated three hours with autoattacks because it pushes you back occasionally.
So I had to restart the game to get out. Which means, I have to restart the story in the Charr kindergarden and have to do the (in my opinion much too long) "get the cubs to safety with Braham" thing again.
Did I just miss something?