Hi everyone, IT'S FIXED for me
I installed the last 1.2.1 update and ran the game. After loading my saved game at the point when I was stuck at the dock with Sigurd, I got notified that the side quest "A hunt for honor" was completed. It was an old side quest that couldn't be validated, probably fixed by one of the updates. It didn't solve the Sigurd issue on the spot though.
I loaded a previous save, from the banquet right before the hidden blade training with the dolls, got the side quest completion notification again, continued up to following Sigurd to the docks, and O miracle, this time I could talk to him and continue the game.

I hope some of you were in the same case as me and get unstuck as well after waiting such a long time.

@Ubi-Woofer I know it's a very complex game and the team has a lot to do so I have been patient enough, but this is beyond anything I could have expected from Ubisoft. 5 months. It's not like it's a visual bug on a sidequest that we can just ignore, it's a bloody game breaking issue at 2% of the main quest, with no workaround. And you've done absolutely NOTHING about it this entire time. Are you even trying? Does nobody care about this? What do we have to do to get the game we bought months ago to work? Do not pretend the devs "are still investigating" because that is a lie.
Shame on you for stalling on this thread while doing nothing to get this resolved.
Shame on Ubisoft for having no regard whatsoever for your customers once you've got the money.
At least I've learned my lesson and will never buy another of your games within 6 months of their release.

@ubi-woofer So you've requested an update 12 days ago. Anything to share?
I'm definitely not restarting the game, losing my progress and spending another 9h, just hoping I won't get the same bug again or a different one at some other point

@Ubi-Borealis
3 months since this bug was reported and we are still entirely unable to play past 2% of the game, WHAT IS HAPPENING UBISOFT?
This issue has been widely documented, you've had dozens of affected players, save games, system files, scenarios to reproduce at your disposal for weeks, and still no news, no compensations, no explanations, no forecasts, nothing remotely useful from your part.
Meanwhile, you've had 100% of our money since day one.
DO SOMETHING. NOW.

@ubi-woofer files attached in case number 13761251
Hope that helps, reach out to me if you need more information

Same issue here, stuck at the dock after following Sigurd. He doesn't do anything. And I can't even do anything else until a fix comes out, because most gameplay options are unavailable during that sequence. How can there not be a fix after a month and a half ? Ubisoft ? Any reaction ?

@guest-zrvvjeif My scenario exactly.
I started the seventh son quest, watched him get killed by a bear, and the quest would never finish.
Then I got stuck when following Sigurd to the docks