Found the cause why the ACA1220 didn't work with the ACA500 and fixed it. The fix is on the ACA1220 side, nothing that the ACA500 could do anything about.
If you already own an ACA1220 and intend to use it with the ACA500 (despite me warning a couple of times that it doesn't work yet), please send it here, and you'll get free shipping for your ACA500 order (wherever on the planet you are).
New ACA1220 cards will work on both, the A1200 and ACA500. I'll contact resellers for any stock they have of the ACA1220, so they will only have the latest version that works on both.