I get the following error:
The port you chose (5903) is not available.
Probably because the VNC server is already running as a Startup Item.
Using canyouseeme.org I see that the port is open.
Using www.gotomyvnc.com I get:
XX.XX.XX.XX is accepting connections on Display 3 (TCP port 5903)
What is the problem?
It sounds like you have a VNC server already running on port 5903. The most likely reason for this is that you have configured a system server to run on that port. Try selecting Server -> System Server… from the menu and see if that’s the case. Another possibility is that you have a second account logged in which is running a server on that port.