Hello James Gibbs-Gerckens,
Thank you for posting in Microsoft Community forum.
Did you have the copy of the certificate within your CAC? Or have you made any back up of this the certificate within your CAC? If so, you can reinstall this certificate to your CAC and then check if it helps.
If there is no any copy or back up for this certificate, how did you get this certificate in the past, you can try to get a new certificate and install this new certificate to your CAC and check if it helps.
If none of them works above, you can try to consult the problem to the engineer of CAC or AF portal and there should be a dedicated engineer to give you a professional and effective reply.
I hope the information above is helpful.
If you have any question or concern, please feel free to let us know.
Best Regards,
Daisy Zhou