Remember the search engine Raddle's server cannot run because it eats too much RAM?
Turns out its Docker container has been running ever since we tested it, and despite being used for absolutely nothing, it's slowly been gobbling up all the free disk space. This caused PHP-FPM to crash, taking the site down.
(FYI, I was given server access again to upgrade Postmill, so I took the liberty of finding out why the site went down. Hope no one minds.)