The app appears to use HTMLPurifier_Config::createDefault() in:
/app/code/application/helpers/html_helper.php

The package should set Cache.SerializerPath to a writable path such as:
/run/easyappointments/cache/htmlpurifier
or /app/data/...

Logs grew up to 21 GB because of the warning below that kept repeating.

/run/easyappointments# tail logs/log-2026-05-01.php 'function' => '_error_handler', 'args' => array ( 0 => 512, 1 => 'Directory /app/code/vendor/ezyang/htmlpurifier/library/HTMLPurifier/DefinitionCache/Serializer not writable. Please chmod to 755', 2 => '/app/code/vendor/ezyang/htmlpurifier/library/HTMLPurifier/DefinitionCache/Serializer.php', 3 => 295, ), ), )