I tried again in Xfce, without rebooting, and it worked.
This morning, I couldn't do it in either session until I rebooted with systemd.
I am still a member of the netdev group, which I believe is the one Network Manager uses. That hasn't changed since I installed MX 19 when it came out.
Is anyone aware of some weirdness between NetworkManager and sysvinit?
(the usual troubleshooting info will follow)
Of course, I try to reproduce it again, and I can't. Everything works fine with sysvinit.
