I suspect you've not followed the instructions to change your output resolution, there's a very good chance that a "quite old" (you've not said what "quite old" actually means, but I'd take it to mean >5 years) TV will struggle with OnDemand in 4K since the last major V6 update.
Home > Help & Settings > Settings > Video output. Ensure 1080 & 720 are selected. Now restart your V6 box.
If this fails, please post the exact model number of your TV, as there could be something specific to this set that's causing issues.
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