When you start TigerVNC server by following command:
then it will automatically bind to port 5901.
When you want to start the server on 5900 then you need to type following command:
This looks a little bit mysterious, but if you dig through the documentation you might find that there is the following mapping:
Now it’s clear that when you omit the parameter, it will default to code :1 which is mapped to the port 5901.