Networking – VNC via SSH tunnel: channel 3: open failed: connect failed: No route to host

I used to connect from A to a system B through VNC with SSH tunnel via C.

I do the following:

1. In B I start the vncserver:

   vncserver
   

   and let's say I get the :12

2. In A I open the ssh tunnel:

   ssh -v -C -L 5912:B:5912 user@C
   

3. In A I start my vnc client:

   vinagre localhost:12
   

But this doesn't work anymore, because when I start vinagre in the terminal associated to my ssh-tunnel I get:

channel 3: open failed: connect failed: No route to host

I know that there have been some problem with the system last week, but I don't know what has changed (I am not the computer administrator), maybe the ip address have been changed but I have removed

~/.ssh/know_hosts

so to avoid problems. I would appreciate any help or hint

I have checked that self-vnc works, by running

vncviewer localhost:12

on system B

I have done some tests, and I have found out that if I try to tunnel and connect to a session open by another user (with another number, like :6) I can connect successfully: in the sense that of course I cannot log in because I don't have the correct password, but at least the vnc client prompts for a password, moreover I don't see any message 'no route to host'. What could that mean? It looks like this vnc thing is not working only on some ports, doesn't it?.

Apparently the system manager set the accessible vnc port range 5900-5910, but the VNC server was assigning me a port number far above this value.