Heh, backward compatibility is definitely something I didn't bother with at all. :)
I've decided against publishing a proper userscript on OpenUserJS/GreasyFork, and instead just put it into my GitHub repo: link.
I'm not fully satisfied with it (some parts are too hacky, and you still have to edit the code to change colors/priorities), but it will do for now. I'm using this version myself, so as long as I am watching movies I'll keep this fork updated.
It's probably IMDb experimenting with enabling site-wide HTTPS. Once you get redirected to a single HTTPS page on IMDb, all other links opened from it are going to be HTTPS too.
Most userscripts are not prepared to work on HTTPS pages, but it's a very easy fix.