@heavyodin53 UPDATE: Started the game up again 13 hours later, was able to speak to village residents and Gunnar the blacksmith, was able to continue the main quest. However, when I got to the point where Sigurd walks to the dock he simply walked up there and stopped. No interaction prompt, nothing. I stood around for a few minutes, and he did nothing. Once again, I am unable to progress the main quest. I did not attempt to complete any mysteries or world events during this time as I had seen on some forums that completing the main story missions would help prevent bugs like this.

It should be noted that this is the worst set of bugs I have personally ever seen in a Ubisoft game. I have been a longtime AC fan, and this is pretty rough. Texture bugs are one thing, and I can laugh them off. But being unable to actually play this game is truly disappointing. Once again, I hope you are able to resolve this in a future patch.

Playing on PC

I have been playing this game the way I usually play Assassins Creed games, which means I like to try to get every objective before continuing the story where I can. However, I have encountered issues with the Mysteries, World Events and Main Quest within Rygjafylke. Here is the current list of issues I have encountered.
The Plight of the Warlock - Both the warrior and the warlock died. I killed the warrior, then the warlock dropped dead. Mystery did not complete
The Lost Drengr in the mountain cave - could not speak with him at all after defeating him in battle. As far as I know he is still on his knee in the cave bleeding out (The y-prompt flashed for a moment, but has not returned)
The comb in the lake under the waterfall - Retrieved the comb, did not speak to the girl beforehand. Cannot speak to her now. (The y-prompt appeared, but after clicking it nothing occurred. I stood there for a few minutes then left)
The Fly Amanita puzzle - I complete the puzzle (As far as I can tell), by going through the three gates. They all flash, the screen whites out, and then I am standing in the middle of the clearing still high as a kite. The mystery doesn't complete
The Main Quest - Family Matters - Speak with Gunnar the Blacksmith - I was able to speak with a couple of people around the village, namely the woman who wanted archery pointers and the man playing Orlog. I was unable to interact with anyone else, including Gunnar. There was no prompt to interact with any of them. This made it impossible to continue the main questline, and as I had already collected almost all the wealth and secrets in the region while exhausting my attempts to complete Mysteries, I was left with basically nothing to do.

This bug/error/issue is gamebreaking. It is impossible for me to continue playing through the game. I hope you are able to use this information to fix these bugs in a future patch.