Can't connect to WP-Admin, blank error message

I have to work on a website, but I cannot connect to the WordPress Administration… I did a forget password and resetted the password multiple times so I know that I have the right username/password combination, but I still cannot connect.

Anytime I try to connect, it comes back to the wp-login page with a blank error message.

Tried to update my WP installation, didn’t fix the problem (site is now running WP 4.2.2)

The blank message appears with both a correct and incorrect login.

I have no idea what might cause this problem, can somebody help?

Heres a screenshot of the problem http://i.stack.imgur.com/eB30X.jpg