Hi, @kimmyx,
The error was on the whole website. Nothing worked until I removed the plugin.
Unfortunelly I can’t update the PHP version, there is an old piece of software on the server that requires PHP 7.1.
But this plugin is marked as working for “PHP version 5.6 or higher”, which is not true, because it breaks the 7.1. If it isn’t marked like this, WordPress won’t allow its update on 7.1.