Just seen this post and I too am having casting issues from the BT Sports app to my Sony Bravia KDL-43W756C and up till 7/10 days ago was fine but now can't watch any sport via the BT Sport app (which i cast through mny iphone to the TV). Have the same issues when I go into the app and cast to the TV, the BT Sport splash screen comes up with V2.1.0 in the bottom right hand corner and the audio starts but it remains on the splash screen and doesn't switch to the actual sport footage.
I've reset the tv, boradband, iphone but still no joy. and again strangely, when I stop the cast, I get the picture momentarily before it shuts down. Went online chat with BT but were not helpful and only said to delete BT SPort app and reinstall and should work, burt didn't.
Welcome to the forum and thanks for posting. Resetting your TV or reinstalling the app won't help here I'm afraid - the Google Cast Receiver on your TV will need to be updated. Can you try following the steps in the other thread to see if that helps you?