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When connecting to an internal website, I'm getting an error: "Certificate contain unknown critical extension." What does this mean?

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I am getting a connection failure when trying to connect to an internal web site:

    Secure Connection Failed
    An error occurred during a connection to <sitename>.
    Certificate contains unknown critical extension.
    (Error code: sec_error_unknown_critical_extension)

Can you give me a general idea what the error is and why I would be getting this error?

Thanks in advance...