Recently, BT gave out an update message stating that your account will not stay logged in automatically. It will need repeated confirmations for you to enter in and gain access, and your BT will appear slightly different. But users are complaining that now their BT mail keeps returning to the login page without showing any error message and it logs in easily but the log in keeps showing up repeatedly without any cause.
A quick fix for this is to delete all of your cache, history and cookies from the web browser. This will prove to be helpful against other issues that you may be facing. Do this by library and menu method. This will result in you having to log in to your My BT and BT mail again. If you still continue to face problem, it is possible that the problem may be related to your browser. Thus, you should try to use a different browser, like google chrome and then check again to see if the problem persists.
Yet another possibility is that you are facing an HSTS issue. To solve this, you can go to your Firefox browser and at the top left corner of the browser window, click on the option of History. Here, click on Show all history. This will lead to the opening up of a pop up box, which is your history library. Then, close all of the Firefox windows that you may have opened up, apart from the history Library that you recently opened.
In the history library, at the top right hand corner, search for ‘ux.btmail.bt.com/cp/index-rui.jsp?v=1.3.14 in the search history box. This URL will then appear in your search history list, unless you had previously deleted your browser history. Right click on this link and in the pop up box that will appear, go to the bottom and select the option ‘Forget About This Site’. Now, close the history window, reboot Firefox and try logging into BT email account again. This should solve your problem