I am in the US (Denver CO) on CenturyLink fiber. I have a Model 3810X Roku with Version 10.5.0 build 4208-50 and my Paramount+ will load but when I select a show it plays the commercials and then fails to load the show and exits. I don't have the issue on my pc or phone on the same wifi network. None of the other apps on my Roku have the issue.
I have rebooted my router, the Roku, and the fiber box but the issue persists.
I removed the app and added it back, didn't help.
I have tried to update both the Parmount+ app and the Roku says there are no new updates.
After removing a channel, best practice is to restart your Roku to clear its memory before reinstalling the channel. Did you do this? Without the restart you may not get a clean reinstall.
Depending on your Roku model and Roku version number you'll find the System restart option under either Settings > System > Power or under Settings > System.
Please give this a try: from you home screen on your Roku, click on settings - click on display type - & select 720p TV. Select ok to use 720p HD, then click "yes, screen looks good." No need to restart anything. See if that works for showing your channels in Paramount+.
Follow the same steps to put your settings back to 1080p.