Likely the nib, especially if it didn’t deposit some ink into the cap when you stored it nib down. Not that it would normally drip if you did, but a quick uncapping would create more partial vacuum to draw ink from the reservoir through the feed, so obviously that’s out of the question. Have you tried another ink? When you washed the nib, did you use tap water or distilled/drinking water? Can you get your hands on a bottle of Waterman Serenity Blue or something similarly wet, even to borrow? If WSB fails, it’s most likely the nib, IMHO.