I only came across Grex in a post last week, so I am a complete newbie on this.
All the Grex inputs/outputs are pre determined, so as far as I can see, it must be down to the ports settings.
I would set up the printer port (LPT1) as port 2 on your system, and make sure that in Port/Pins etc, you define which port you want that function on. The pin settings should be as normally allocated.
I do not know whether you can perate the printer port togeyjer with the ethernet connection.