Mhhh, this is really strange. The call works for me (after hardcoded the non-default port in the ssh_func.inc) so not sure whats going on here.
As an alternative you could also do a “full” scan to gather the necessary information. For this you need to clone the “Full and Fast” scan config, go to the Network Vulnerability Test Preferences and change the setting “Enable SSH Debug” of the VT “Global variable settings” to “yes”.
Afterwards a full scan with this scan configuration should log the required information to the openvassd.dump (GVM-9 and below) / openvassd.log (GVM-10+). The location depends on your installation but should be somewhere in /var/log/openvas.
Again, thanks for providing all this info. 