Pocket Beagle <=USB=> Ubuntu 18.04 networking problem.

I am trying to set up a couple of Pocket Beagles from a Ubuntu 18.04
workstation, but the USB network devices are not showing up. I know that the
hardware is good (cable, etc.) since everything works with an old Mac Mini,
but I want to use my Ubuntu desktop. I'm thinking that something is missing,
maybe some udev magic or a missing package or something like that.

OK, even weirder: things work fine on my laptop, also running Ubuntu 18.04.
The only difference is that my desktop is set up to support several virtual
machines and sets its main Ethernet interface with a static IP and folds it
into a bridge to allow the VMs (KVM/QEMU via LibVirt) to access my LAN

sauron% less -X /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto kvmbr0
iface kvmbr0 inet static
        # dns-* options are implemented by the resolvconf package, if installed
        dns-search deepsoft.com

while the laptop has the stock interfaces file:

gollum% less -X /etc/network/interfaces
# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback

I just installed and started NetworkManager on the desktop. No difference.