The URL I posted describes a way to serve the resonse page to users without TLS decryption.
The firewall is able to see the URL - at least the FQDN - in the TLS handshake even without decryption. But by default the firewall is not able to inject the response page into this connection as this is only possible when the firewall sees the actual http traffic. So try the solution in the url and hopefully this will solve the issue for you/your customer.
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