Para resolver esto, se debe activar en /etc/nxserver/node.conf el debug en nivel 7:

NX_LOG_LEVEL=7

Después de conectar, veremos un fallo similar a este:

Info: Auth method: passdb ssh The system has no more ptys. Ask your system administrator to create more.
while executing
“spawn -noecho $command_ssh -2 -x -l “$user” “$host” -o “NumberOfPasswordPrompts 1″ -p “$port” “$executable $command” ”
invoked from within
“if { “$auth_method”==”ssh” } {
set pid [spawn -noecho $command_ssh -2 -x -l “$user” “$host” -o “NumberOfPasswordPrompts 1″ -p “$port” “$executable $…”
(file “/usr/lib/nx/nxnode-login” line 61)
FREENX> 716 Slave mode failed to start.
Info: Closing connection to slave with pid 21998.

NX> 404 ERROR: wrong password or login
NX> 999 Bye

Para arreglarlo, cambiamos los permisos en /dev/ptmx a 666:

chmod 666 /dev/ptmx