There were some recent Kernel updates in Qubes. In the Settings | Advanced for the TemplateVM if I switch from the (new) default 5.13.6-1.fc25 kernel back to an earlier version 5.12.14-1.fc25 then things start working normally again.
If I leave the AppVM based on this template at 5.12.14-1.fc25, then it works as well.
It seems that this particular Debian 10 template is the only one that breaks with the newer kernel. This template has some Snaps and other software installed. Other Debian (full & minimal) and Fedora templates appear to be working.