Hi, I also have a Microserver Gen 8 and was having the same ACPI Error message spamming issue as well.
I found THIS POST and went ahead and unload the "acpi_power_meter" module from the kernel and it solved the problem. You can add this line to the /boot/config/go file found inside the flash drive hosting unRAID system to have it unload the module during every boot:
rmmod acpi_power_meter
Hope this helps!
p.s. I found another article from Novell which also sort of explains this situation.