I just started seeing the same error screen that the OP (bollyboy) reported on May 25.
I know I have the right password because I can login to the iCloud.com page and look at my address book, but when I click the mail icon I get the "can't load mail" error screen.
In addition I can't send or receive mail with this me.com account in mail.app or on my iphone. Everything was working fine until this morning.