I have the same problem. In my mac mini (late 2012) with i5 and 8gb of ddr3 ram, v6.8.3 worked fine.
I upgraded to 6.9.3 and was having trouble booting. same error message.
I do not think it is the fault of the USB stick, it has always worked, but anyway I tried with others (even new ones) but nothing to do.
Mem test correct.