Thanks for reporting this.
I "fixed" this issue by commenting out line 274 (return false;) on the new version 1.48. I'd need to take a better look at what happens on the IMDb page when there are no custom lists to deal with the situation properly, but that means deleting all my lists or creating a new IMDb account and looking over the code, but I'll leave that for when I have more free time.
I recently deleted the only list I had on IMDb and the script stopped working for movies in my ratings list and watchlist. If I create an empty list it starts working again.
I did some digging and found that deleting lines 271-274 of the script fixes this.
Thanks for your work!