After deleting an import from Letterboxd (as have been suggested in this post), my letterboxd watchlist has been secretly kept, making it impossible for me to remove a movie/show from the watchlist.
It can be related to this post.
Attached is a video from the behaviour.
Have you tried resetting your browser and progress data?
That usually helps with such issues. Something with cache.
Thank you @lifeiscrazy , it worked perfectly.