After updating PHP to versions 7.1.12 and 7.0.26 Wordpress plugin NextGEN Gallery stopped working.
The issue occurred because of PHP bug https://bugs.php.net/bug.php?id=75573.
The issue is resolved in PHP 7.2.0 which was released in Plesk Onyx 17.5 in the scope of Update 33.
Note: NextGEN Gallery plugin update which disables some functions which lead to crashes was released on December 1. Update plugin to the latest version.
Otherwise, as a workaround, change PHP handlers for the Wordpress site to PHP to 5.6 version:
Home > Subscriptions > example.com > Websites & Domains > PHP Settings.