I was having this problem, as well as intermittent com port errors, until I replaced the USB to Serial cable.

I'm using a MacPro running 10.5.6
Parallels 4.0 with a virtual of Windows XP SP3
MX-850 Editor version 1.12.288
IO Gear's USB to 2-port Serial RS-232 Converter, Model # GUC2322
Gender changer
Serial cable that came with the MX-850

I previously used (without success), a Prolific HL-340, and a Belkin USB to Serial. The prolific kept getting the:
"Error detected while communicating with your Remote Control.
Error code: 100-37UU-0000
Either the com port setting is incorrect (Communications Menu) or the cable is defective."
Well, I had two cables of the same model, and both got this error. Time to replace the cable, so I bought the Belkin F5U409.

The Belkin got rid of the 'com port or cable' error, but then I got this error:
"Invalid Answer - LimitedProgramLoadToPC [DLTR-2]"

So, I bought the IO Gear GUC2322, and all is well. I have read, programmed and re-programmed my MX-850 over 50 times since I replaced the USB to Serial cable, and connections under Parallels and VMWare Fusion is working flawlessly.