Motor Wiring Error, Popped controller

I built a gokart using a 28v 65a 2 hp shunt field motor. I used a series field motor controller. Note a mismatch. I noted an oddity with my voltmeter during the initial testing, but choose to ignore it.
The kart ran great with with wiring scenario A, we drove the heck out of it for 4 days.
Please view attachment of wiring scenarios.
Then I got curious about the voltage oddity. After some testing and wire tracing I came up with the improper shunt connection in wiring scenario A. The shunt is connected to B+ on both ends, one end through a 20 ohm resistor.
I corrected that with scenario B. The gokart did not run as fast so I dumped that scenario.
Looking at scenario A, I noted the shunt has no battery current through it so I tried disconnecting it, as in scenario C.
This was a mistake, the gokart shutdown twice in in 1/4 mile, the second time never to run again. The motor controller is defective, it gives an "overvoltage error code". Error codes are supposed to be self clearing but it doesn't.
So the obvious answer is to get the motor controller repaired and reconnect as in scenario A.
But scenario A is not really proper.
I'm looking for a reason the controller popped and the best connection for this mismatch.
Thanks, Mike