Captive Portal and certificates

Hi everybody, I have an issue with the Captive Portal on a Fortigate 100E (6.4.4 1803) configured on a WiFi network (Cisco). The Fortigate is connected to a local AD Server (Windows Server 2016).

The authentication on the portal works well unless for the fact that after a user authenticate itself, he got a an error:

The page isn't working

192.168.1.254 didn't send data

ERR_EMPTY_RESPONSE

Opening another tab on the same browser, the user can navigate so the authentication seems fine.

On this firewall I also have a certificate issue. It hasn't been provided a proper certificate from a CA so I'm worried that this could be the issue. If it is, is there any temporary workaround?

Thanks.