Just thought I'd post this to help anyone experiencing the same thing I did.
As has been advised recently, ISC2 have changed their SSO provider and as of today members are being prompted to go through a process to update/reset their passwords as part of the move over to the new system.
I just went through this process but unfortunately it was not so straightforward for me. When I initially logged in, I was prompted as expected to go through the password update/reset process, which involves following a link sent to you by email.
Due to the security settings on my email account, the link in the email was being mangled by a URL rewriting mechanism causing me to go into an endless loop of requesting a password reset, receiving an email with a link to follow, which would only take me back to the page to request a password reset, and never actually be prompted to perform the reset.
Once I figured out what was happening, I temporarily disabled the URL security feature and was able to go through the process to update my password and login.