modx manager login fails

When I type my manager username and password it fails silently, despite having worked in the past. ie. no error shows up, it simply goes back to the login page. I have submitted a password reset, then used those credentials with the same result. I have gone into the user table via phpMyAdmin and modified the username and password, with the same results.

I'm wondering if there is some system setting that has changed or been corrupted.

Hoping you can help.