That's usually a local problem, I just checked and it worked for me.
What you need to do is open the dominion site in a private (Firefox) or incognito (Chrome) Tab. That should give you the login screen. Fill in your credentials, but do not click "Login". Instead, look below, there is small text reading "Kick" and "Kick & Resign". These are actually clickable links.
Click on "Kick & Resign", that should clear the problem up, and you can continue in your normal browser window/tab. If you are in a rated game that you don't want to resign from, it might be a good idea to try "Kick" first. If that works, fine, othwerwise use "Kick & Resign".