I have an 8 port serial card that uses Netmos-Moschip 9835 chips. My PIC programmer will not work on any of the Netmos ports, but will work just fine on the two serial ports on the motherboard. On the other hand, a UsRobotics v.Everything modem works just fine on the Netmos ports.
I think it is an RS232 signaling issue. Proper RS232 is specified as a current loop having +- 12v signaling levels. However, many interfaces will work with +-5v, and so some inexpensive devices do not bother generating +- 12v. I know my PIC programmer does not employ an RS232 interface chip, and I rather think the modem provides a serial interface that is fully RS232 compliant. I suspect that my PIC programmer, and probably your device, is taking some "generally acceptable" liberties with the RS232 spec that are not compatible with the Netmos-Moschip chips, but since my programmer works fine on the motherboard ports, I haven't had the need to determine exactly what the issue is.