Some other users are also seeing this error with other shows on Paramount and apparently Paramount is aware and working on a fix. Wouldn't hurt to contact them as well and add yourself to the list of affected users.
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When this happened previously, Paramount reportedly just whitelisted some IP addresses rather than address the underlying issue.
Unfortunately, you're wrong, because FBI plays just fine on the same network using a phone, Chromebook, and PC.
@Boz it's meaningless if something works on a different device. First, the app itself is not the same as the one on a Roku. Same thing if you're using a web browser, completely different method of playback. Second, the larger service providers, such as Netflix or Paramount, feed different devices from different servers so there could be a server issue for the provider.