I bought a db9 to 3.5mm stero adapter cable and was having the same communication errors.

I was able to find a way to swap tip and ring on the cable and everything worked just fine.

Measure the voltage at the end of the stereo plug with the adapter connected to the computer. If the voltage (9v) is on the tip, you have the right cable, if its on the center ring, then you need to swap them. (db9 null modem would work, or kluge it like I did!)