OK I fixed the problem. I was using Keycaptcha and for some weird reasons, the captcha wasn't showing on login page.
I used the browser console as suggested Dave to see what was going on, and the fact that my website was using SSL was blocking the loading of a captcha script (mixed content).
So for those using Keycaptcha, edit the class_humanverify_keycaptcha.php file and replace the http with https and that should work fine