I have been having the same problem, There was a tracking cookie called adviewmanager driving Yahoo nuts... I ran Malwarebytes, cleaned all the cookies out and low and behold it’s perfect now... Give it a try, Malwarebytes is free!
Sorry, Wrong software, Clean your cookies out with Glary Utilites.. My bad... It will find all your cookies and deleat them for you...
My bad, Sory for the bad information!