It might be a problem with logging in with either your posting key (the private one) or the active key (also the private one). You can find them both in your wallet on Steemit, under permissions I think. Try either of those BUT NOT THE MASTER KEY. Never use that unless you have lost access your account.
That's all I could think of as being the issue. That or the site just has a bug which is not unlikely. Happens a lot on these platforms since it's all very new. Or you could login via Steemconnect, which is kinda like login with facebook. Again, you'll need to know what password you should use (posting or active key) for that too...
I might be totally off though. I use a password manager, so I dont actually remember if they are at all different. Good luck though!