Although it used to work fine, YT no longer works on my Sony Bravia running Android TV - or to be more accurate, works but won't log in to my account.

Last year they removed the ability to log in using Brand Accounts, however that was restored and the version of the app on my TV should support logging in with a Brand Account, which all of my Google accounts are. However I cannot sign in. It just refuses to allow me to do it. It used to just be signed in with the main Google sign in but now isn't and does not seem willing to allow me to input specific log in details separately.

I reinstalled the YT app to see if that made a difference, but it did not.

Anyone have any suggestions? I can't seem to locate useful support online.