This on two different Windows/virtualbox machines that have been successfully completing ATLAS applications.

I tried a reboot, but that didn't help. No Atlas tasks are being downloaded.

Any clues?

The queue of ATLAS-tasks is empty: https://lhcathome.cern.ch/lhcathome/server_status.php

To be more specific, why is the host telling me that there are no usable WSL distros found when I am not using WSL; I am using Virtalbox.

Maybe I am getting too old to keep up with current technologies.

This is part of the newer BOINC versions.
On your systems with BOINC 8.2.8 BOINC wil check during startup, whether WSL is available, just like it checks whether VBox is available.
When WSL is availlable it also checks which (default) distro is used.
In the (far) future the BOINC-team will probably switch from VBox to WSL containers.

WSL containers and VirtualBox can't be used at the same time on a host, cause VBox don't run when Windows Hyper-V is enabled.
Only Theory can make use of WSL at he moment. If you want to run all three (Theory, ATLAS and CMS) stay with VirtualBox.