the guide should apply to WiFi just as much as wired - you do the captive portal settings in SSID instead of the interface, but that's about the only difference. I assume from your statement the FortiGate is redirecting to FortiAuthenticator, but instead of the FortiAuthenticator login page opening, the browser displays 'ERR_EMPTY_RESPONSE'?
Can you double-check the portal policy in FortiAuthenticator? It needs to have two different IPs set - the IP FortiGate uses as RADIUS client (usually the interface it uses to communicate with FortiAuthenticator), and the IP/hostname of the interface it uses to redirect to captive portal (SSID interface IP).
In addition, you can check the following in FortiAuthenticator: - go to https://<fortiauthenticator>/debug - select 'RADIUS Authentication' in the drop down -> FortiAuthenticator treats captive portal authentication attempts as RADIUS against itself, so you might see something like 127.0.0.1:x -> 127.0.0.1:1812 and NAS-Identifier including the string 'FAC_GUEST'
Aside from that debug, double-check that you have an HTTPS server certificate set on FortiAuthenticator, and that captive portal is allowed in the FortiAuthenticator interface.
If the issue persists despite this, I would suggest opening a ticket with Fortinet Technical Support for some in-depth troubleshooting.
+++ Divide by Cucumber Error. Please Reinstall Universe and Reboot +++