I'll try all your suggestions in a little bit and then get back to you.

The page Oliver links to mentions that the earthmate uses pins 4 and 9 for something, which I have removed. What does "PC DTR" (pin 4) or "RING" (pin 9) mean and are they needed for this to work?

I am currently running hijack 334, which looking through its history didn't change the serial port options yet. I'll upgrade to the latest version and set the menu entry just in case though.