It seems you're not the only one
https://www.amazon.com/forum/home%20...x25CR202H580T7
Physically unplugging the TV and plugging it back it seems to be a fix.
Recently I had a problem with 4oD on my H series TV. I had reset the Smart Hub to get it to work.
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Also I can't quite work out whether it's still available or not. For some apps they only support a certain year onwards, but the list on Amazon's website isn't the best:
https://www.amazon.co.uk/gp/help/cus...54E32EAED2BA8B