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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.