In the PPM 15.3 New UX, clicking on the 'Forgot Password' link does not work as clicking it does nothing. The user isn't brought to anywhere to change their password and instead remain on the login page.
This is caused by defect DE38147.
This defect is resolved in PPM 15.4.
Have a PPM Administrator reset your password for you. You can then use that new password to log in.