Possible fix: APU GCU
Tip: In the main E&E look at the #1 GCU (in this case we had a BTB Fault light illuminated). Start the #1 engine and see if the generator will come on line and extinguish the Transfer Bus off light. Then when placing the aircraft back on APU bus 1 power, look to see if the APU bus 2 stays on line.
Explanation: This problem seems to come up at 5 minutes to push when the APU is started, or is noticed then when the crew is doing their flow.
Applicable Reference: FIM 24-21 task 810, 24-41 task 801
Thanks to Tom S. for his input !