The problem#
I use Ubiquiti-branded network products for my switches and wireless access points and use the UniFi Network controller software to configure them.1 I noticed the web interface wasn't working and checked the status the service and saw the logs showed it starting the service and no further information (and looking at the log files didn't show it generating any errors either):
$ sudo systemctl status unifi.service
[...]
Aug 16 14:32:13 host systemd[1]: Starting unifi.service - unifi...
Specifically, this was on version 9.0.114-28033-1 of the unifi Debian
package provided by Ubiquiti.
The solution#
After multiple steps of investigation, I finallly got the Unifi service
running by changing the ownership of all of its data files to the
unifi user/group:
sudo chown -R unifi:unifi /usr/lib/unifi/{data,logs}
But this might not have been the best fix: the first thing it did when I opened the web interface was tell me there's no database and offered to restore from a backup. Luckily I don't change the settings much and it did regular automatic backups (in addition to a slightly older manual backup I had), so I didn't lose anything. Also, it's possible the data loss was due to some other troubleshooting step, not that one. As always, keep backups and before making a change consider making an extra copy of the directory before the change (or making a snapshot if you're using ZFS or another filesystem that supports them).