Hello,
he FW enables ports 80 and 8443 to provide the HTTPS service by default. When you use the web browser to access port 80, the FW automatically redirects the access to port 8443 for you to log in through HTTPS.
Before changing the service port number of the web server, run the undo web-manager [ security ] enable [ port port-number ] command to delete the configured service port. Otherwise, the change will fail.