Post mortem
Freeflarum and its hosted forums were down on January 18th, 2018 from 6:03 to 7:05 UTC.
Cause
The cause was a MariaDB file descriptor limit, kicking in due to the number of hosted forums (360).
Solution
Increase FD limit in OS settings, systemd settings and MariaDB settings. Merge with deployment tasks for future servers.
Meta
Monitoring worked as expected. At 6:03 the application level monitoring detected the first anomaly. At 6:04 the external service monitoring confirmed the outage. It took 12 minutes to identify the cause, then 15 minutes to implement a fix, 10 minutes to validate the fix and 3 minutes to merge the fix in the deployment system.