I am having the same problem. The solution didn't work for me. Still getting intermittent problems. Sometimes I can sign in, other times I can't. All the problems have arisen recently. I have a btyahoo main account with two 'proper bt' email addresses. One day I expect my btyahoo address to be migrated to 'bt proper'. I am trying to sign in with this web address and have tried the solution but both give the same result:
I spoke to BT. There are no staff available, it seems, to thoroughly investigate and BT will attempt to tell you it's a problem with your computer.
I usually use Edge browser but managed to connect on my PC via Chrome by using this method (but not every time). 1. Sign in AFTER waiting for the wheel to stop turning on the "sign-in" web address header. 2. If a page comes up saying Can't connect to Web Page then wait. You will see a wheel turning on the web page header every now and again as it seems to automatically refresh. With a bit of luck you will eventually connect on one of the refreshing attempts.