I am also experiencing an issue with Pluto not playing on Roku. I noticed it first one one device, selected Pluto like I always have and I was presented with a dark screen and then after about a minute, it bounced back to Roku home. The problem manifested to another box in our home and now it's out on both boxes.
I tried pulling power from the box, that did not work.
I tried reconnecting to the wifi/internet, that did not work (other apps work fine).
I tried removing the Pluto app and re-adding it, that did not work.
I tried it on my phone and Pluto is working.
Appreciate any thoughts.