Mainly it is happening when users sign out of their profile, they then cannot sign back in again. As an experiment I have signed out of my old PC from IE, been unable to sign back in, changed to firefox and my id was already there, signed out of that one, couldn't sign back in, Basically there is no relationship between the problem I had on there and the browser, it was the same message about not using anything for a name but letters and numbers.
If I hadn't actually typed in my user id, because it never got to that screen, how did it know what I was going to type? unless the problem is related to my specific user id and using the . in the middle.
but that sounds too human a thing for any programme or interface or whatever to anticipate, shirley?