I've reset the Roku, uninstalled the app, reset the Roku again, reinstalled the app, and reset the Roku yet again to no avail.
My network on the Roku is my Xfinity home network.
I only seem to be getting this error on HBO, and even then, it only seems to occur with select titles/series.
Any help would be much appreciated.
It seems that you are being shown this error for not being subscribed to HBO. If you wish to see content from these premium channels, you should look into adding it to your current plan.
More information here: https://www.xfinity.com/learn/digital-cable-tv/premium-channels/hbo