realvnc protocol version 3.6 is required
After googling a bit, I remembered that the protocol used by RealVNC (and VNC) are now compatible with an RFC that is used by other software like Virtual Server. Both use the same port, 5900,
The target computer had this software installed and when RealVNC was installed, the service just binded to the second port, 5901. To access this port using the RealVNC client, you just have to select the second view, using the :1 command after the host/ip.
-f.