The storage volume behind box filled up. Uploads and downloads failed with 500s; clips couldn't save new replays and the home page's box card errored.
A parser bug in clips wrote demo files to box in a loop, filling the 200 GB volume in ~40 minutes. There was no quota alert, so the first signal was the outage itself.
Loop fixed, orphaned files purged, volume alerts at 80 / 90 / 95% added, and box now refuses writes at 97% instead of falling over.