The "SEC_ERROR_UNKNOWN_ISSUER" error message typically indicates that the browser is unable to verify the identity of the website's SSL certificate's issuer, which is typically a certificate authority (CA). This can happen for a few reasons:

  1. Invalid or untrusted certificate authority: The website's SSL certificate was issued by a certificate authority (CA) that is not recognized or trusted by your browser. This could happen because the CA is not in your browser's trust store, or because the certificate was revoked by the CA.
  2. Self-signed certificate: If the website is using a self-signed SSL certificate, your browser will not recognize it as a valid certificate because it is not signed by a trusted authority.
  3. SSL interception: If you are on a corporate network, it is possible that your traffic is being intercepted by a device that is configured to examine SSL traffic, it will reissue a new certificate for the website being accessed which is not trusted by your browser, leading to the question mark icon. In this case, you would need to contact your IT administrator to check if this is the case and how to proceed.
Here are some steps you can try to fix the issue:
  1. Verify the certificate details: Check the certificate details to make sure the certificate is not expired, and it was issued by a trusted CA.
  2. Add the CA to the browser trust store: If the browser is not recognizing the CA, you may need to manually import the CA's certificate into the browser's trust store.
  3. Clear browser's cache and cookies: Clearing your browser's cache and cookies may fix issues related to SSL certificates.
  4. Disable VPN or proxy: If you are using a VPN or proxy, try disabling it and accessing the website again.
  5. Update your browser: Make sure that you are using the latest version of your browser, as older browsers may not be able to access websites using the latest security protocols, and they might not have the updated trust store.
  6. Check your firewall settings: Make sure that your firewall or antivirus program is not blocking the connection. You can temporarily disable them to see if they are causing the problem.
  7. Download the certificate: In some cases, if the CA is not in the browser's trust store, you can download the certificate from the website and manually import it into your browser.
  8. Contact the website administrator: If you are still unable to access the website after trying the above steps, you may want to contact the website administrator and inform them of the issue. They may be able to help you resolve the issue or provide you with an explanation of why the certificate is not recognized.

It is important to be cautious when you receive an SSL error, especially when it comes to the issuer, If a website presents a certificate from an unknown issuer, it may be a sign of a man-in-the-middle attack and should not proceed.