Hi,
Based on my experience, to Configure User Certificate Autoenrollment we have to configure the user based policy under: Default Domain Policy, User Configuration>Policies>Windows Settings>Security Settings>Public Key Policies>Certificate Services Client - Auto-Enrollment.
So we need to make sure the users have received the Auto-Enrollment policy .
Based on your description :b) I've checked to ensure the workstation in question has received the GPO's for certificate auto enrollment.
The policy *Computer Configuration**>Policies>Windows Settings>Security Settings>Public Key Policies>Certificate Services Client - Auto-Enrollment is just for computer to Enroll certs automatically not for the users.
We have to configure the Certificate Services Client - Auto-Enrollment policy both under the user configuration and computer configuration.
If i misunderstand you , please feel free to let me know.
Best Regards,