If you are on the website and try to
in rare cases an error message "Access token expired" can occur.
Known cause: The URL you get has a limited lifetime. For registration and password reset, it's 1 week. For link to Steam it's 6 hours. If you do not click on the link within the lifetime, the error occurs.
The unknown cause (only Steam link): A lot of people claim the get the error even within the lifetime of the token. We believe that, unfortunately we cannot reproduce it and therefore don't know the reason.
In case 1) just retry: Re-register, re-request a password reset or restart the Steam linking process.
In case 2) in many cases it helps to logout and relogin to the website.