@ubi-woofer do the following to try to reproduce the error (bc it seems the players have experienced this):
1 Early in the game, just after Eivor save her friends and flee from the island, do not go back to the raven's camp. Stop at Stavanger.
2 Once there, go to the side quest of the seventh son of a seventh son who wishes ro kill something. When he starts to celebrate a victory, let a wolf kill him. This will bug this side quest and nothing can be done.
3 Now, go to the camp. Above the camp, there's a side quest of a lady who wishes a comb in the water. If you have the problem of most of the players, you can't talk to her and, if you try, the game will bug.
4 Now complete the main missions of the area, including the blacksmith and the seer, until reach the Sigurd's return.
This script seems to be the one players have done. Some did more of the side missions and treasure quests, but others did only this.

They just answered my support ticket. My platform is PS4 and they sugested to take actions by the MSCONFIG, like cleaning the temp archives. OMG, it's just absurd, outrageous, ridiculous. Ubisoft shows absolutely no respect for it's consumers.

@guest-gyfilpqf
Did the same. Didn't complete all mysteries and treasures yet, but so far so good (and i have done more than before). But still, the problem remains. Imagine we get far in the campaign and then, boom, new bug that stops the progress (and maybe, considering how much we have played, maybe turning back isn't a valid option)? Ubisoft must solve the problem. It's shameful, disrespectful.

@ubi-swaggins omg, it's a few days now without any notice of solution about the problem. We are not your beta testers, show some respect to the people that bought your game.

Same here, but on PS4.
What have i found so far and that maybe related, according to the other answers:
1 - Mystery of the guy who wants to kill something: the wolf killed him and the icon remains. Nothing eles happens.
2 - The NPC from the comb mystery doesn't answer. Pressing the dialog button bugs ALL dialogs with any other NPC. Reloading the game solves (except for the comb lady, who remain mute).
3 - The prodigal prince (main mission): after the assassination training, following Sigurd is useless from the moment that he reaches the dock. Reloading, even to a point before the familiar subject main mission, is useless. Nothing seems to fix the problem.
Link to the vídeo of the final bug (prodigal prince):

Having the same problem here, but on PS4. Similar problem on world events, with NPCs that does not answer.