I created a new collection for the Oliver Stone movies Wall Street. I was able to add a backdrop by clicking on the green + above posters (all other means to add a poster do not work). Now when trying to add the individual movies to the collection using the section "Parts" I get a blank error and the movies do not get added.

Collection: https://www.themoviedb.org/collection/354702-wall-street

When trying to select the first Wall Street movie the select list looks like this:


When trying to select the second Wall Street movie the select list looks like this:


This is the error I get: