Same's been happening to me, during the Nemnok fight.
Save file is here.
Here are the steps to recreate the bug:
1. Talk to Nemnok, agree to deliver the grimoires.
2. Walk over and take the quarterstaff from the middle chest, near Nemnok.
3. Everyone attacks, at some point the whole party is downed except Pallegina, who is at 0 HP yet still has her battle cries and the fight doesn't end no matter how much damage is done to her.
Weirdly, I had another save called "Nemnok Fight Bug", from right before the fight, after the talks are done, which I intended to submit. But now when I load that one, the fight works as intended. I'm still linking it, in case it turns out to be useful.
Also, here are some screenshots of the situations. The last one shows active effects on Pallegina, who is the only one fighting with no health in this instance (just recreated to test it).
The first two screenshots are older from when I first got the bug Eder and Xoti are alive with 0 HP in them too. Also, that "-2147483648" string pictured in the second screenshot is the same one OP is getting; I'm guessing it's related to this particular bug.
OP, try loading the save from before entering the barrows and going in with a different set of companions. Nemnok fight worked for me eventually, but I used a different approach (picking an [attack] option through dialogue).