I have been getting this message on my webmail page for about a week now.
We're experiencing a temporary problem with Email and are working to fix it as soon as possible. Please try again later. Sorry if this causes any inconvenience.
When will this be fixed
All I can suggest is that you read some of the other threads on this board, and see if you can find someone with the sam problem.
It works fine here, using the Firefox web browser, have you tried a different web browser?
nobody with same issue after btmail switch, plenty of people with the issue when using yahoo mail. However these answers are out of date as they refer to the old system
Try using an email client such as Thunderbird or Windows Live Mail. It might not be the most suitable solution for you but while BT try and sort this out you should be able to access your email.
see link for setting to set up an email client.