Which certificate are you using? Go to Security Services/HTTPS Proxy...
If you're using the "Demo certificate" that came with the WSA, you can click on the "Edit Settings" button, and click on "Generate new certificate and key", and then re-distribute that cert to your workstations via Group Policy....
If you're using a cert from your own CA, you'll have to go there and get a new "Subordinate signing certificate" and upload that to the WSA.
