SOPHOS UTM9 Home Edition, Server 2016, HTML5 VPN and "Error: Protocol Security Negotiation Failure" error HTML5 connection over UserPortal.

 

https://community.sophos.com/sophos-xg-firewall/f/discussions/86373/user-portal-rdp-connection-protocol-security-negotiation-failure

https://community.sophos.com/utm-firewall/f/vpn-site-to-site-and-remote-access/75897/solved-i-get-the-following-error-when-i-try-to-connect-to-my-server-over-html5-portal-error-protocol-security-negotiation-failure

Changing the Protocol Security: RDP to TLS

https://community.sophos.com/utm-firewall/f/vpn-site-to-site-and-remote-access/78192/error-protocol-security-negotiation-failure-error-html5-connection-over-userportal