No emotions involved in my proposal to Jacques - we've put time and money into solving this issue, but couldn't solve it for him. The best I can do now is to offer a refund. There's supposedly two other solutions for the A500 out there, and they might work better for him.
The CF card slots on the ACA500 are unbuffered. The "unbuffered" property explains why some CF cards keep the ACA500 from starting at all. The bus towards the A500 has data bus buffers, but no address bus buffers. If the A500 has some non-standard expansions (kickstart switcher, memory expansion?) installed, the address bus may have unusually high load and the 68EC000 can't drive it any more. This may be different if you attach an A1200 accelerator, because the address bus will then be driven by the CPU on the A1200 accelerator.