The current version of the Pluto TV Roku app does not work with Roku 3 hardware / firmware (or anything of a similar or earlier vintage).
EMail to their "support" link results in a canned reply that (paraphrased) "The problem has been fixed by Pluto TV engineers and uploaded to Roku. Try reinstalling to get the latest version." This has been their standard "response" for months now.
Any additional information or questions sent in reply is ignored. There is never any follow-up.
Removing / reinstalling the app is the ONLY way for it to work again, and that only survives until you close the app / open another one.
I can only conclude that:
1. Pluto TV has written off anyone not using the latest Roku hardware.
2. Pluto TV has no real "support" department, but likely just an overworked developer or two who might check email and social media on occasion to make sure the app isn't dead for everyone.
Alright, I've accidentally found a trick: Hit OK on the remote a couple of times. Sometimes responds when I hit it only once, other times requires the second click. I've done it enough over the past few days to make sure it ALWAYS responds after touching OK.