Are you sure the .jpg in .thumbnails is valid? Have you opened it in a picture viewer?

Obviously removing (1).jpg won't help. However removing Pigs_Can_Fly_Etc*.mp4 and .thumbnails/Pigs_Can_Fly_Etc*.jpg followed by a new download might. As might renaming (1).jpg to Pigs_Can_Fly_Etc.jpg and moving it to .thumbnails, overwriting the one already there.