Thank you for your quick responses, but the problem remains unresolved despite all your advice.

Same cause as Heyado7018, disconnecting the "Honeycomb Bravo" makes the APU work, why?

I may have found one of the reasons for the APU malfunction. In the "Controls/Instruments and Systems/Electrics" menu, key 42 on the Honeycomb Bravo was assigned to the "APU Starter" command. I removed it, and it works.