When I try to log on to either email or MyBT, i get the message 'strongauth.secure.bt.com says ca is not defined' the screen then halts and I have to try again. This can sometimes occur 2 - 3 times before a successful log on
I have this same problem. If I right-click on the webpage and hit 'reload' it then works properly.
Try clearing your browsers history and cookies. If it lets you, select the option to clear from all time.