Any chance this could be related to the serial port arbiter extension thing on some older macs, or maybe even if you might have Appletalk ON but not meaning to? Might also be an IAC thing holding the port open.

I'm guessing Appletalk has grabbed the port, in which case make sure you go and turn that off if you haven't, and also make sure you set your desktop printer to something for sure not on your free port. Sometimes a serial direct printer can setup shop there and show that way.

Very interesting problem you have here.