It looks like the vncserver is listening on 5901 not 5900 - basically unless you ask for a specific port / display number it chooses the next free one
What I typically do is ssh in and then start vncserver on a specific display e.g.
(it will complain if display :5 / port 5905 is already in use) then you *know* which port to open (5900 + display #)
vncserver -geometry 1440x900 :5
In fact I'd recommend tunneling VNC over SSH - you still need to know which port/display you started the server on in order to set the tunnel end(s) - but you *don't* need to mess with your actual firewall / open ports because regardless of the VNC port, everything goes externally via your (fixed) SSH port.