This is something I noticed signing in just now. Upon entering 'Matrim's Dice' into the display name box and my password into the password box, I received the message alerting me that the display name or email address I had typed did not belong to any account. I did type it right; after double checking the spelling and everything it still gave me the message.
However, when I entered my email address in the box I signed in just fine.
This isn't as much of an issue as it is a report, I figure if something like this happened to me it's probably happened to others. I don't know how often this would happen; I typically don't sign out and sign back in. I'm curious to know if this is something that's happened before to anyone else, or if it was a fluke mishap on my end.