I'm a happy owner of an ACA500plus, and have been successfully using it on few of my Amiga 500s. A while ago, I connected it to an A500 Plus that I just bought. Everything worked fine, without one of my AmRAM-16 512KB memory expansions plugged in. Once I attached the memory expansion, the Amiga started going nuts. Amiga Test Kit crashes immediately, all WHDload games crash and some crash with a red screen, Workbench desktop "jitters" and has graphical glitches.
All of my regular A500s still work perfectly fine, with the ACA and memory expansion attached. My A500+ works with ACA, without the memory expansion, or with the expansion but without ACA.
But now, in Amiga Test Kit's "Direct Memory Scan" portion, there's this warning "12 holes in memory map??", that wasn't there when I first started using the ACA. This warning shows on all of my machines now.
Has my A500+ somehow messed up the ACA?