I started removing plugins based on their update dates. I removed only one:
CA Mover Tuning
...and I was able to restart.
I then saw there is an update today for the Unassigned Devices plugin that I still had installed. This is in the release notes for that update:
Fix: Deal gracefully with udev not returning any information on a partition.
Fix: Issue with loss of GUI on 6.9.
Add: Show a disk that drops out of the array and ends up as an unassigned device as unmountable and mark it as 'Array'.
Fix: Rewrite check for used share name because the scheme used interferred with a device mount.
I wonder if the "Loss of GUI on 6.9" is my issue.
In any case, I updated to it and was able to reboot again. I'll mark this SOLVED for now.