If you're using 3rd party password manager application, such as LastPass, you might run into issue where you need to press "Sign In" button twice in order to log in.
- Type in username + password
- Press "Sign In"
- You see the progress indicator flash beneath the user name text box shortly, but nothing else happens
- You press "Sign In" the second time
- You get authenticated and logged in
Keep pressing that Sign In until you're in.