I too have suddenly hit this problem after casting without problem for the last nine months since I took out BT mobile subscriptions.
I have super fast FTTC broadband running at the full 50 meg and two BT mobile (6 gig data) subscriptions.
Help would be appreciated.
Looking at my BT account then there is no longer any mention of BT Sport on my account. When I go to 'Manage BT Sport' the site wants to start an order???
If you got the BT Sport free because one of your BT Mobiles has 6GB data or more, then casting is not supported, you have free app only access, if you've previously been able to cast it was in error, most likely a recent change to your package fixed that.
If you have BT broadband, then you can upgrade to the chargeable BT Sport app, that supports casting and multiple other devices like an Xbox, PS4, certain Apple TV's ect
Suspected that might be the answer, thanks, from reading other posts.
Don't understand what I was getting it and what triggered it to stop.
If, on my BT Broadband subscription, I buy the 'chargeable app' do I still need my chromecast device, If I do will that allow me to cast from either of the two BT Mobiles please?
Sorry should have said that TV is a Samsung smart TV