The error show only when I attempt to access by VPN. There is no problem when access from internet.
Access Error: 401 -- Unauthorized
Interface is not authorized for HTTP access
In "Security > Zones/Screens". "Internet" zone which include the below interfaces were set to allow "https, traceroute, telnet, ike and ping".
fe0/0/0.0 <- direct connect to modem
st0.0 <- tunnel for the VPN
Both fe0 and st0 were set to allow the same services. Why only VPN not allow?
The error message said "... not authorized for HTTP access". But I access by "https://xxx.xxx.xxx.xxx". Shouldn't it show HTTPS instead of HTTP?
Problem solved by after read the KB you provided. Add the tunnel interface under "service web-management". Don't know why the security settings in GUI was different to CLI.