I experience this problem since just a few days. Usuallly I get all my movie metadata and covers from the movie database agent - but now it won't get any artwork anymore. If I try to enter the poster url manually it won't work either, as long as it is a https:// adress. When I chance to just http:// it loads the artwork. But that is a hell lot of work.
I just wonder why plex won't load the artwork automatically anymore. It did work for me before. I kinda changed nothing on my machine.
Any solution to that?