The issue can be closed.
The globus-gridftp-server had 2 problems:
it used a incorrect hostname for the gridftp server. It used the hostname and not the hostname on the external interface. It therefore used the wrong interface to setup connections. Fixed by adding: $GLOBUS_HOSTNAME to /etc/gridftp.conf
It used the wrong ports for the TCP range. The ports where therefore disabled in the firewall. Now it is set OK:
listing hangs
When broken of the logfile shows:
What did I do wrong? It is authenticated etc..