Just use Gmail instead, BT are no longer offering email to new customers, the same goes for some other ISPs.
Try deleting cookies and internet history (cache I think it's called) and see if that works.
I use my email address as log in to pretty much everything I use online - I've had my btinternet email address for 15 years+ and have never had an issue with it
FWIW, I bookmarked the old link https://email.bt.com/mail23/index-rui.jsp?v=MX_2.32.0#app/mail , and it has been working fine for days now - no delays, no problems keeping signed in, etc. And I haven't had to delete any cookies.
I'm using Chrome with Windows 11, all up-to-date.
HTH
Have you seen any improvement? I've seen a couple of posts that seem to suggest the problem has gone away?
Thanks
Roget
You are posting on a year old thread so hopefully the other posters can now access their account.
If you are still having problems you would be best to start a new thread instead of jumping on an old one.