OK, I'm progressing : the ssh issue seems to be gone.
I changed some parameters but not sure which one are relevant.
When comparing with a fresh install, here are the diffs :
root:/etc# diff /etc/ssh/sshd_config /tmp/sshd_config
< #PermitRootLogin yes
< #AllowTcpForwarding yes
< #GatewayPorts no
The man page specifically says that the -R option won't work unless
GatewayPorts is set to yes, and it is NOT able by default.
But i don't remember if this was sufficient, because I may have
change PermitRootLogin in the same time.