NET:ERR_CERT_INVALID despite SSL decryption being enabled.

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You have enabled SSL decryption on the proxy, but the SSL service on the SSL error page still displays a certificate error

This is a common error, as we often try to use the certificate used by the proxy in the Web console's SSL service.
However, the proxy certificate is a root certificate, so that the proxy can generate certificates on the fly for the visited sites.
It must be the same for the error page service. 

You need to go to the certificate section to generate a website certificate with the proxy certificate.

• On the certificate center section, click on the used certificate by your proxy service
• Select the “Secondary Certificates” tab
• Click on New server certificate button

• Pay attention on the Subject Alternative name, it should be the hostname to your Web error page.
• Click on Create button
• You have now a new server certificate based on your the Root certificate used by your proxy.
• Back to the Web error page settings and modify the used Certificate to this new server certificate ( with “Server:”' suffix)