I found a wiki page advising VirtualBox users to disable "I/O APIC", and to change the virtual network adapter to "PCnet-FAST III". But this made no difference.
Just to be thorough, I re-installed completely from scratch with these settings. Still no difference.
Just to be crazy thorough, I created a new VM profile that was set for a 32-bit guest. Reinstalled from scratch. Still no difference.
Gave up.
Nothing but respect for the ReactOS folks, and all they've done over the years. But I imagine that the overwhelming majority of people who try this out are doing so in a VirtualBox VM. So if you don't have an up-to-date and accurate list of instructions for installing ReactOS on VirtualBox, prominently displayed near the download page, then you might consider prioritizing that.