I have the same problem on the United Forever page.
I tried old passwords and new and only receive authentication errors.
https://player.trackmania.com/main.php
I am still supposed to be using the credentials I use to sign into the TrackMania United Forever game itself, correct?
Thanks in advance for any help.
EDIT: Newb fail! Last time I changed my password, I fat-fingered it the same way twice, so I had an incorrect spelling. After recovering the password, I changed it... but the player page still won't let me log in. Is there a time that I need to wait while the change propagates to the player page website as well?