The error as reported in post #5 is that the certificate for the DPT site has been revoked.
A SSL/TLS (technically x.509) certificate can be revoked by either the CA (root Certificate Authority) or intermediate, this is normally achieved through the issuing of a CRL (Certificate Revocation List); CRLs are distrubuted downstream from - but this does not guarantee that all intermediate CAs, proxies etc get them.
It's not unknown for CRLs to contain errors... and are quite a clunky method of checking the status of a certificate. Most moders systems/orgs will now use OSCP (I'll avoid sending folks to sleep with an explanation of how that works

)