I have this issue on a newly installed perfect server with ISPConfig 3.1.7p1, Debian 9.2, Apache 2.4.25 and PHP-FPM. Action: example.com/non-existent-file.php Browser output: "File not found." Error.log: [proxy_fcgi:error] [client] AH01071: Got error 'Primary script unknown\n' Expected: Handled by ErrorDocument (like any other file types). Another issue, I cannot run PHP files in the default /error folder of a website anymore. The source code is sent to the browser. Thanks for helping out Edit: this post should probably have been posted in the Server Operation forum. Sorry for that.