On Friday November 15th we had a short Phabricator downtime.
All php-fpm child processes were used by it.
Disabling the aphlict service (and puppet) was the immediate fix and brought Phabricator back normally.
For now aphlict is disabled. Users will still get normal notifications but no realtime pop-up notifications.
There is a theory this is related to recent changes on the caching (ATS) side because we did not have this problem before and it's not a new setup.