As it lacks the month it's supposed to be ranking in. Introduced by Iba8d0d14f1adef08195dc5bb7dfbc.
Fixed! Link now works: https://tools.wmflabs.org/awmd-stats/month-rank/2019-03. It was failing as monthID was changed to month_id and but didn't update the template correctly so it was undefined in the Jinja template.