Searches generate error 'CiviCRM_API3_Exception: "Constructor failed"'
On the surface this was a memory issue - increasing PHP memory limit from 512mb to 900mb fixed the error and let search work as before.

The most likely cause was the ICU update for PHP 7.2.16 - http://site.icu-project.org/download/64 - which came out &amp; was merged into FreeBSD the night before the problems emerged. I can't really take it any further than that. The ICU update referenced "Various Out-Of-Memory (OOM) issues have been fixed" so maybe that's a clue if someone else had the same problem and can't increase PHP memory limit further.