How to remove the certificate errors for webadmin and captive portal authentication pages

Hi TobLai, Thanks for reaching out to Sophos Community.

What did you set as the 'Common Name' while creating the self-signed certificate and how do you access the firewall? Via IP address or by any fqdn? 

Common Name I typed in the IP Address and then in the Advanced settings for Certificate ID, I use IP address .

Common Name I typed in the IP Address and then in the Advanced settings for Certificate ID, I use IP address .

Did you install the CA in the local machine? Also, Check the certificate presented by the WebGUI from that Local Icon before the URL.

If possible, share the snapshots of the browser certificate and your self-signed certificate configuration