What are the More Uncommon Causes of Error 2101-0001?

Hi there, I need some insight on the 2101-0001 error code. In most cases it’s caused by a faulty M9 IC paired with a bad charging port. What about for the cases when M9 and the port are not at fault?

I’ve got a switch here that has proper voltage on pin 5 and 6 on M9 and it charges fine with both a 5v and 15v charger. The error also came up with a charged battery and no port installed. I was told that Pi3, audio IC, and the BGA just above the audio IC can cause this error code.

I’m curious as to what you guys think. Any help is greatly appreciated.