I'm having this exact issue all of a sudden. When prompted for user/pass, I enter the correct information, the progress bar comes up momentarily, then disappears and I'm back at the sign-in prompt with no errors, but I'm not signed in...
I can direct my browser to the exact same link and sign in with no problems, so I know it's not my information that's wrong. Also, I am not using a proxy page. I was using this sign-in function previously with other services (which no longer exist) without any issues.