Did you try forcing a check in Settings or were you just waiting for it to auto-update?
Did you try rebooting the Fire TV? I'm not sure if you are actually using URL blocking (as you say) or just domain name blocking (as what is typically descri...
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.