I think since they changed the button “Add to list” some days ago to “Add to watchlist” the whole adding stuff to lists is bugged for me
When I try to manage what lists is something added to, it says 0 of 1 sometimes in the page of the show/ episode/ movie (when i have multiple lists) and when i press the icons is like it freaks out and it sends me to my Dashboard and says “you are already logged in”.
Can you send a screenshot or extra details of where you’re seeing this issue? That would help us or others work out what the problem is or if there’s a workaround/solution 
else try this too:
I had the similar issue of not being able to add to my watched list, then i tried the web version and there was an error explaining that i need to upgrade to a premium version for adding more than 100 movies to a list. so i created a new list it worked fine.