I have encountered an unending combat bug a couple of times on regular fights. Reloading and finishing the fight again solved it. But this time was different:
I accepted both Galawain's and Rymrgand's quests. First I went to Noonfrost and persuaded the pale elves to walk away. I sealed the breach. I had killed 1 guardian before that since I wasn't stealthy enough (I got too close). Note that I didn't find the prisoner of the Prison of Ice quest at this time, since I got out of Noonfrost fast, thinking that there still might be guards or such (there weren't).
Then I went into the Maw to do Galawain's quest. I killed some lions and lion tribesmen, and after reaching the big lioness, I decided to kill the bear and so I did. Exploring the place further, I got the said Prison of Ice quest, re-entered Noonfrost and released the prisoner.
Now, after re-entering the Maw, I suddenly found myself in combat mode, with no enemies in sight. This made it impossible to finish Prison of Ice.
Solution: Yeah, the good news is that upon loading the latest autosave (inside the Maw), I found out that the "combat" didn't start! So, I finished the quest normally, phew.