Do you mean a web server certificate for SSL (HTTPS) connections to the web server; if so that is nothing to do with the LANDESK ServiceDesk installation and is more IIS and installing a replacement certificate on your web server in the DMZ.
Or do you mean Token authentication?
thanks for your feedback.
We had an issue where the STS token on the DMZ had dissapeared (support and myself were unable to verify how this happened). The solution which worked for us was performing a restore of the DMZ server to a previous "correct" state,
After performing the restore, the token was back on DMZ and users were able to login to the system (Workspaces + Webaccess).
Thanks for your feedback.