After rebuilding my desktop, VirtualBox worked fine except that my Windows VM could not connect to the shared folder I had configured for it. After finding this page, I was able to run this command:

VBoxManage setextradata "Windows 10" VBoxInternal2/SharedFoldersEnableSymlinksCreate/vmShare 1

where "Windows 10" is the name of the VM, and "vmShare" is the name of the share, then restart the VM, and it worked. Obscure thing, so I document it here. The issue is that the path to the shared folder depends on a symlink on my system, since the shared folder lives on an encrypted data drive, not the system drive.