I most cases there no reason to change this port number, normally you use port forwarding in the web server software to connect for example external port 80 to run time port 8080
What kind of server are you running on? Opening ports below 1024 may be restricted.
I just looked this up, error code 3 means you've likely chosen a port number that is already in use. This seems not to be documented.
Anyway, you cannot use a port below 1024, because those are only available when running as root, which I hope you certainly are not doing with your Mendix application process.