After the seasonal event started the longboat disappeared when in England. It can be summoned, but it's invisible along with the crew. When summoned in port it's stuck and can't be moved even though player can take command. Regardless of the summoning position, the sail fold/unfold order does not work. When starting a raid - still possible with invisible longboat - crew does not join the fight. Problem persists in England, despite fast travel, sleeping, modifying the ship or progressing the game. It does not affect the ship in norway or the river raid boat.
The problem concerns multiple save files. Reloading very old save made the longboat appear again, but it's always invisible on all newer ones.

@ubi-keo the problem still persists after the last update and with updated graphic drivers. What changed is that this time sail unfold option worked. You can check the video under the link: '>

Can we expect an asnwer from development team any time soon? It's been almost a month and still no word. This is a game breaking bug and it still persists.

Another month passed without a single update. Is the development team even investigating the problem or did they just give up?

I'm so glad that I found this discussion. I'm having the exact same problem. Longboat is invisible, and when I press "e" to take command I become invisible too and I'm unable to do anything but press "tab" or "esc" to fast travel or load a different save. On older save files the boat still shows. The thing is that this was my only save file of this playthrough in which I'm trying to get 100% completition with almost 100 hours of gameplay with fully completed 11 regions (including Asgard and Jotunheim) and every mastery challenge with gold medals. I really hope this can be fixed.
(I'm sorry in advance if I butchered the English in my comment it's not my native language)

I found a solution! It's a bit tricky, because I did it with my PS5, but it was pretty simple: I got AC Valhalla on the PS5 (it was a bundle, so it was basically for free). When I launched for the first time it told me my pc and playstation accounts were syncronised. I loaded the save file from my pc (the one I talked about in my previous post), and to my amazement the longship actually appeared. Then I turned off my PS5, and loaded the save file on my pc and it was fixed.

I know this isn't a great solution for other people like @voli1o, since you need to buy the game again, but this might be helpful to find a solution in a future update.