A momentary/occasional "channel not authorised" which clears after a couple of seconds of changing channel is one thing, but it's if the message doesn't self-clear, that's another story.
You're right, the internet connection is for OnDemand, EPG updates and other stuff - your live TV channels come over the co-ax cable. Have you tried doing a channel refresh on the affected box? Home > Help & Settings > Help > TV Care > I'm missing some channels.
When you say "downstairs the box is fine", do you mean that the downstairs box is fine? Or that this box, is ok in another location?
If the issue follows the location, this suggests a problem with the internal cabling. If the issue follows the box, then there's your answer.
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