It seems that the WPA (and WPA2 especially) implementation is differently handled by different vendors. Some vendors do seem to have the same setup, but you cannot assume anything.
There are quite a few threads in this forum on problems trying to get a WPA[2] connection set up correctly.
Mostly the advise is to keep all hardware the same brand/vendor to circumvent this issue, or lower to WEP, as that standard is much more smoothly implemented. I personally would not like to go lower than WPA2, but in some cases you might need to.
Downgrading firmware is not really the solution, but in some cases it could help. You might need to read the reason (release-notes) of the firmware-upgrade, and I cannot help you on the location for older firmware. Could you contact the author and let him supply you with the (location of) the firmware?