Reserved Port
Why This Error Occurred​
Server was started on a reserved port. For example, 4045 is reserved for the Network Paging Protocol (npp).
next start -p 4045
or
next dev --port 4045
Starting the server on a reserved port will result in an error.
Possible Ways to Fix It​
Change the provided port to ensure it's not listed in the Port Blocking section of WHATWG's fetch spec.