My records show that my password did NOT have a special character in it. When I tried to log in from the browser where the supporter ID and password were cached, it worked. In a browser where they weren't cached, it would not let me in (saying "invalid password").
After changing my password to include a special character, I can sign in from all browsers.
[Same was true for partner's account, so don't think this was me cocking up.]
When you request a password reset, be patient for the email to arrive. Don't do like I did and request another password-reset email, as this means that when the first email eventually turns up, the link in it will lead you to the password change screen, but it will not accept any valid new password. Following the link in the second email did allow the password to be changed.