PC only powers USB devices after moving to Japan. What should I check?

Start by reseating the RAM. Then try booting with one stick at a time in the motherboard's recommended slot. If one works, add the others back one at a time and try different slots if needed.

The reported fix in this case was a RAM seating or configuration issue. All four sticks eventually worked after reseating and swapping them around.

Something may have shifted during transport even if it looks fine. With the PC unplugged, check the motherboard power connectors and reseat the graphics card and its power cables. A heavy card can be a particular trouble spot, including if it was reinstalled after the move.

Strip the setup down for testing: motherboard, CPU, one RAM stick and integrated graphics if the CPU supports it. Otherwise, use the graphics card. Disconnect nonessential drives and peripherals, then see whether it starts.

USB power only tells you that some standby power is present. Likewise, a spinning fan in the paperclip test doesn't prove the PSU can supply stable power under load.

Check the input rating on the PSU label rather than assuming the travel adapter changes voltage. Japan uses 100 V mains, with 50 Hz or 60 Hz depending on the region. A supply rated for that input should not need a voltage converter.

Since you've already tried a local cable directly at the wall and the supply is rated for the input, I'd look inside the PC next rather than keeping the extension cord in the test setup.