Per Fujitsu support recommendation, I replaced main board. Same issue! It booted fine until B9 on the screen, then threw the same error. Unable to make it boot that far anymore. Even when having components on the table and only empty main board, PSU and the PSU cable card connected, it just clicks, blinks green on PSU and then gives up.
What is even more strange is that even with a totally separate (non-fujitsu) PSU, it just clicks and refuses to start. With such separate PSU it attempts to start every five seconds or so and fails, while I believe Fujitsu PSU is smart enough to only attempt it once as it gets some error. And this same behavior on both old and new main board.
Anyway, looking at the "show resolutions" in iRMC log, I saw this:
At system power on sequence the PSU only provides the standby voltage
Check for overtemperature of the power supply
Check for overcurrent in System Event Log
Check for short circuit
Retry to power system on.
Disconnect AC and wait 30 seconds before reconnect.
Replace power distribution board if installed otherwise replace PSU"
And surprise surprise, it recommends replacing the power distribution board. Amateurish Fujitsu support telling me to switch main board.
Now ordering this part next.