Here's a fix:

1. switch to dual wield hammers
2. stand on the wall beside the Drengr - I was on the collapsed side
3. attempt to melee him (it will miss) using heavy hit
4. at the same time press triangle repeatedly, even if there is no icon
5. after a few tries at this, the dialogue started!

Now the new problem I have is that every territory is 100% complete, but the final two achievements won't pop...

@ubi-borealis this bug is now 5 months old - what priority does it have on the dev team's backlog?

https://www.reddit.com/r/ACValhalla/comments/ju1f9c/glitch_unable_to_talk_to_lost_drengr_after_defeat/

@grymislegend did you ride the horse up onto the wall or what?

I just came back from Norway and the weird thing with Sigurd and Basim, and the only story arc I had left was Hamtunshire. At Hamtun Blockade I released the prisoners. I spoke with Guthram again, and he talked about forces gathering at Chippenham. The quest is complete, but Holy Day has not been offered, and there are no markers on the map.

I have the same problem. He's stuck on a low wall. I was wielding a pair of 2h axes and I think I used the kick ability.