What happens if you use this link?
If it takes you directly to the BTMail logon page use it to make a desktop shortcut and use that in future.
Try the following to see if it helps.
Log onto your BTMail email account using a browser.
Once logged on and on the main email page with the list of emails, go to the webpage address bar and save it as favourite.
Log out of your email account and close the browser.
Open the browser and click on the BTMail favourite that you have just made. This should take you to the log on page.
Log on as normal.