Fairly simple and easy to reproduce. In the combo creator you cannot select any button other than "A". You can click on the "A" button, and the dropdown list showing other keys does appear, however no matter what you click, it will not change from "A".
I tried manually editing the weapon file with a text editor, but since I couldn't figure out what value the other keys were, I only managed to turn it into a "-1" combo, lol.
Customer support service by UserEcho