I dutifully removed the regolith packages and re-installed them, only to get the same error as before.
After removing them a second time, I noticed that the packages were listed in `apt` in the `residual config` state, which Debian uses to indicate that the old config is still hanging around.
A search and test later, the following command helped to remove all packages in that state:
for i in `dpkg -l | grep ^rc | cut -d" " -f3`
sudo dpkg --purge $i
Then my system was ready to reinstall and run error-free once more.