Can you share your ArcaOS 5.1.x start string for qemu, version of qemu, and host os you are using?
These are not mine, but here's some start strings found around the web, for versions of OS/2:
- ecomstation 2.x:
qemu-system-x86_64 -m 2048 -device vmware-svga,vgamem_mb=4 -device pcnet,mac=6E:C0:E6:AA:6B:BD,netdev=net0 -netdev user,id=net0 -drive file=ecomstation21.qcow2 -device ac97 -rtc base=localtime -cdrom ecs21_cd1_en_us.iso -boot d
- ArcaOS 5.0.x:
qemu-system-x86_64 -m 8192 -device VGA,vgamem_mb=4 -device pcnet,mac=6E:C0:E6:AA:6B:BD,netdev=net0 -netdev user,id=net0 -drive file=arcaos50.qcow2 -device ac97 -rtc base=localtime -cdrom ArcaOS-5.0.iso -boot d
Source:
https://www.reddit.com/r/qemu_kvm/comments/1gek98d/install_and_run_ecomstation_21_and_arcaos_50_in/Other resources:
os2world.com/wiki/index.php/Installing_ArcaOS_on_QEMU_with_Windows_10_Host
ArcaOS 5.1.0 was first version to support UEFI? So, perhaps some UEFI investigation, to fallback to CSM or something else, for QEMU? Or, as you are running a licensed 5.1.0, perhaps open a ticket with ArcaNoae?
Hope this helps ...