I also had the same issues. However, I was able to fix it by deleting "/system/app/SuperSu.apk" and then side-loading an older version of SuperSu (2.01). So, it looks as if the issue may be with compatibility with more recent versions.
Back to my answer...
Disclaimer: I'm new to this whole Android thing, so what I'm writing
this based on my own research and experimentation. So, take things with
a grain of salt: I may be off base on some of my conclusions.
Anyway, I've ...
Looks like Plex is available for free on the Amazon App Store for Fire TV. Don't know for how long, though. It's normally $4.99.