Authentication not working for Netflix on Apple TV 3

I know it's getting old now and the only mainstream services still available on the Apple TV 3rd gen (at least here in Europe) are Prime and Netflix, but they're the only ones I use anyway. Except Netflix no longer works, either before or after applying Apple's latest software update. The problem? It won't accept my login credentials. Says it doesn't recognise them. They work fine on my MacBook so it's not the credentials. It returns 'ATV-ui92' each time I try. It could be a regional issue because I can use someone else's (Polish) account on this same ATV3 although Netflix thinks that's unlikely. Seems impossible to delete and reinstall Netflix so I can't try that. The ATV is not behind a VPN. Any other suggestions gratefully considered. Thank you