HTTP 500 Error class_wp_query

Hi Experts,

i have a HTTP 500 Error. This is the Message in the Error LogFile

30.09.2017 19:23:43 mynvt.de [client 94.31.85.0] PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 28672 bytes) in /mnt/web010/e2/69/5139169/htdocs/WordPress_06/wp-includes/class-wp-query.php on line 617

So i have checked the class_wp_query.php but it looks like that there is no error.

        foreach ( $keys as $key ) {
        if ( !isset($array[$key]) )
            $array[$key] = '';
    }

Can anyone help me?

Thanks in advance.

Kind Regards Léon Zimmermann