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Apparently Pluto has been making some changes to their channel which broke access for many Roku owners recently.
The good news it has now been fixed and is once again working. But you need to do this first.
Here is the Fix for Getting Pluto TV Working again On Roku:
1. Remove the Pluto TV channel from the Roku home screen. Do this by navigating to the channel icon (but don't enter the channel) then, press the * key on your remote control. From the menu that pops up on your screen, choose 'Remove channel'.
2. Restart your Roku player by navigating to Settings>System>System restart.
3. Finally, after your Roku has fully restarted, re-add the channel back again by clicking here. (Remember to check for updates again and afterwards the channel should be back on your Roku).
This is all it will take and it should get Pluto TV working once again on your Roku. And let you enjoy all the great new content that they recently added to their channel.