After working into the Apache configuration, I found the problem. The apache module mpm_prefork module was enabled, but the configuration file /etc/apache2/conf.d/hestia-event.conf, which handled the php-files, was configured for mpm_event_module:
<IfModule mpm_event_module>
<FilesMatch \.php$>
SetHandler "proxy:fcgi://127.0.0.1:9000"
</FilesMatch>
</IfModule>
I just disabled mpm_prefork and enalbed mpm_event. Everything find now.