Below is the full error message that client received:
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Expired SSL Server Certificate (ssl_server_cert_expired)
Your request contacted a host which presented an expired or Invalid certificate
This is typically caused by a Web Site presenting an incorrect or invalid certificate, but could be because of a configuration error.
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When bypassing the ProxySG there is no error.
Checking the validity of the certificate when browsing the website without going through the Blue Coat SG seems to be ok, the certifcate is still valid.
The issue is due to the certificate been sent by the server. The certificate that expired is actually the CA certificate that signed the server certificate.
This type of issue is not proxy related, the administrator needs to check with the server administrator regarding to the CA certificate that had expired.