I have noticed an issue in the latest build.
It is not sufficient to simply close a browser tab when exiting a user panel. If you fail to actually logout then you will not be able to log into another user panel, unless you re-log into the user panel that you failed to log out from, and then log out before closing that tab (or logging into the next user panel)!
You can usually see which user panel you forgot to log out of by looking at the bottom of the left (menu) margin of the admin panel.
If all else fails, then completely closing the browser (all windows) and then restarting the browser will let you log in.