you might want to maybe try a different distro image to verify, maybe a simple kernel with a net image or something.

This part actually makes me wonder… Do you think SerenityOS uses the Linux kernel? Because it does not, it's its own completely separate thing. And the hardware support for anything other than the standard emulated machine is very iffy, so it doesn't seem too surprising that it would get tripped up by something on an old computer.

If anything went wrong with its USB stack for example, the kernel would have no way to find the root filesystem that's stored on a USB drive.