Try some of the troubleshooting steps on the megathread page: Megathread for MineOS webui issues
Most notably, it seems likely your server is running, and running properly. Based on your error message, though, one or more of the links to complete the connection are stunted.
The aforementioned megathread should allow you to run commands on the MineOS host itself, which can be used to confirm operation.
- check via terminal that the host itself is listening on the port (
netstat) - check via desktop (non mineos server) ping connectivity works
- check open media vault if 8443 is being NATted