USB stick stalls boot

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I am trying to install pfsense CE on an old "TERRA Black Dwarf UTM" device. It has an old Intel u3500 CPU with the default x64 instruction set. When I copy PFsense to a USB stick (pfSense-CE-memstick-2.4.5-RELEASE-p1-amd64, copied with dd), and attach it to the machine, the boot will stall (it just hangs at a certain point). I tried with several USB sticks (which varies the final point of the stall) but it is always somewhere in the splash-screen of the bios.
I have seen such things with wrong HW architectures but in this case it seems correct.

Any hints on how to debug it?