Before plugging on of my ACA1233n (26 and 40MHz versions), I was running stable 28MHz on my ACA500plus, everything working just fine.
When plugging in the ACA1233n and using it inside the ACA500plus boot menu, everything works fine as well (26Mhz or 40MHz depending on the card I have.
The issue arises when I chose to disable the ACA1233n in the boot menu, returning the Amiga to the host 68000 processor. The machine does not boot in 28MHz at all (it gives a 'D9' error on the DiSMo). When I use 21MHz, it boots just fine, and when inside workbench, I can press the DiSMo button to increase the speed back to 28MHz and everything works ok.
I have tried this in both my ACA1233n cards and also on different A500 machines and all reproduce the same problem.