This KB throws light on a few ports that should not be used for the HTTP/s Management page. Creating a custom port for HTTP/s management is often recommended practice. Configuring a custom port for Management page is explained in the below KB article:
How to change the HTTP and HTTPS management ports on UTM Appliances?
There are a few ports used that are reserved for internal communication between SonicOS and SonicCore. When you use these ports for the Management page, it throws an error "Invalid port" or "Reserved for SonicOS use".
CAUTION: This restriction is only for Management port, and NOT for any port forwarding or other inbound access