@veskis I've got the exact same problem. I think it's the guard from the highest section where two start at. By the time I've got to that area there's only 1 guard left and no body in sight and I've not already killed him. Going to try some other possible fixes and I'll let you know if anything works.

[Edit] Managed to kill all the guards by engaging the middle 3 guards (1 crossbow and 2 archers up high) after the first guard on the right. I'm not sure if the guards dropping to their death rather than you hitting/killing them is what messes it up. I then had some headshots not register despite clearly being headshots and 1 shot kills. Stupid game.

[Edit 2] Just managed to complete it. There's 5 guards on the left hand side. The one next to the cage I would suggest killing first out of them lot as he glitched under the map a couple of times for me if he was alert and trying to move towards me. Pain to get it done but it is achievable with the glitches.

@crewchief00206 try quick travel to the spot right next to the keep. I did that and it reset the characters positions and I was able to progress with the quest.