Posting on behalf of my dad who's in his seventies. He's using IE11 and every time he tries to watch live sport full screen, he gets an error message saying Internet Explorer needs to close. He's rang BT Customer Care a few times and they just tell him to use IE11 (which he is) and change his BT password (which he has, although I doubt this has any impact!).
I having being having similar with IE11 but not only with BT Sport but with BBC iPlayer. I have taken to using Firefox for both which works fine so I suspect the problems lies with IE11. I suggest setting up a desktop shortcut for Firefox / BT Sport to make it a doddle to get to.