FastCGI: incomplete headers

Tema Divi 2.5.6 y todos los plugin actualizados. Bastantes veces me sale en la web de un cliente el error: FastCGI: incomplete headers. Veo la cabecera de errores del fichero log de mi servidor y además de este me da los siguientes:

http://stylopiel.es/producto/billetero-rocio-lunares/
[Wed Nov 04 19:00:23 2015] [error] [client 137.135.176.145] FastCGI: incomplete headers (0 bytes) received from server «/var/www/vhost/stylopiel.es/fcgi-bin/php.fcgi», referer: http://stylopiel.es/producto/billetero-rocio-lunares/
[Wed Nov 04 19:00:23 2015] [error] [client 137.135.176.145] File does not exist: /var/www/vhost/stylopiel.es/home/errors/500.html, referer: http://stylopiel.es/producto/billetero-rocio-lunares/
[Wed Nov 04 20:17:04 2015] [error] [client 83.51.12.155] (4)Interrupted system call: FastCGI: comm with server «/var/www/vhost/stylopiel.es/fcgi-bin/php.fcgi» aborted: select() failed, referer: http://stylopiel.es/producto/billetero-rocio-lunares/
[Wed Nov 04 20:17:04 2015] [error] [client 83.51.12.155] FastCGI: incomplete headers (0 bytes) received from server «/var/www/vhost/stylopiel.es/fcgi-bin/php.fcgi», referer: http://stylopiel.es/producto/billetero-rocio-lunares/
[Wed Nov 04 20:17:04 2015] [error] [client 83.51.12.155] File does not exist: /var/www/vhost/stylopiel.es/home/errors/500.html, referer: http://stylopiel.es/producto/billetero-rocio-lunares/
[Wed Nov 04 21:15:04 2015] [error] [client 95.121.146.235] (4)Interrupted system call: FastCGI: comm with server «/var/www/vhost/stylopiel.es/fcgi-bin/php.fcgi» aborted: select() failed, referer: http://stylopiel.es/producto/bandolera-serraje-beige/
[Wed Nov 04 21:15:04 2015] [error] [client 95.121.146.235] FastCGI: incomplete headers (0 bytes) received from server «/var/www/vhost/stylopiel.es/fcgi-bin/php.fcgi», referer: http://stylopiel.es/producto/bandolera-serraje-beige/
[Wed Nov 04 21:15:04 2015] [error] [client 95.121.146.235] File does not exist: /var/www/vhost/stylopiel.es/home/errors/500.html, referer: http://stylopiel.es/producto/bandolera-serraje-beige/
[Wed Nov 04 22:00:43 2015] [error] [client 79.108.57.119] (4)Interrupted system call: FastCGI: comm with server «/var/www/vhost/stylopiel.es/fcgi-bin/php.fcgi» aborted: select() failed, referer: http://stylopiel.es/wp-admin/edit-comments.php
[Wed Nov 04 22:00:43 2015] [error] [client 79.108.57.119] Handler for fastcgi-script returned invalid result code 1, referer: http://stylopiel.es/wp-admin/edit-comments.php
[Wed Nov 04 22:00:43 2015] [error] [client 79.108.57.119] File does not exist: /var/www/vhost/stylopiel.es/home/errors/500.html, referer: http://stylopiel.es/wp-admin/edit-comments.php
[Tue Nov 17 16:01:35 2015] [error] [client 79.108.57.119] (4)Interrupted system call: FastCGI: comm with server «/var/www/vhost/stylopiel.es/fcgi-bin/php.fcgi» aborted: select() failed, referer: http://stylopiel.es/
[Tue Nov 17 16:01:35 2015] [error] [client 79.108.57.119] FastCGI: incomplete headers (0 bytes) received from server «/var/www/vhost/stylopiel.es/fcgi-bin/php.fcgi», referer: http://stylopiel.es/
[Tue Nov 17 16:01:35 2015] [error] [client 79.108.57.119] File does not exist: /var/www/vhost/stylopiel.es/home/errors/500.html, referer: http://stylopiel.es/
[Tue Nov 17 17:17:06 2015] [error] [client 87.219.212.93] (111)Connection refused: FastCGI: failed to connect to server «/var/www/vhost/stylopiel.es/fcgi-bin/php.fcgi»: connect() failed, referer: http://m.facebook.com
[Tue Nov 17 17:17:06 2015] [error] [client 87.219.212.93] FastCGI: incomplete headers (0 bytes) received from server «/var/www/vhost/stylopiel.es/fcgi-bin/php.fcgi», referer: http://m.facebook.com
[Tue Nov 17 17:17:06 2015] [error] [client 87.219.212.93] File does not exist: /var/www/vhost/stylopiel.es/home/errors/500.html, referer: http://m.facebook.com
[Tue Nov 17 17:17:10 2015] [error] [client 79.108.57.119] (111)Connection refused: FastCGI: failed to connect to server «/var/www/vhost/stylopiel.es/fcgi-bin/php.fcgi»: connect() failed, referer: http://stylopiel.es/wp-admin/admin.php?page=wpseo_advanced
[Tue Nov 17 17:17:10 2015] [error] [client 79.108.57.119] FastCGI: incomplete headers (0 bytes) received from server «/var/www/vhost/stylopiel.es/fcgi-bin/php.fcgi», referer: http://stylopiel.es/wp-admin/admin.php?page=wpseo_advanced
[Tue Nov 17 17:17:10 2015] [error] [client 79.108.57.119] File does not exist: /var/www/vhost/stylopiel.es/home/errors/500.html, referer: http://stylopiel.es/wp-admin/admin.php?page=wpseo_advanced
[Tue Nov 17 17:17:24 2015] [error] [client 87.219.212.93] (111)Connection refused: FastCGI: failed to connect to server «/var/www/vhost/stylopiel.es/fcgi-bin/php.fcgi»: connect() failed, referer: http://m.facebook.com
[Tue Nov 17 17:17:24 2015] [error] [client 87.219.212.93] FastCGI: incomplete headers (0 bytes) received from server «/var/www/vhost/stylopiel.es/fcgi-bin/php.fcgi», referer: http://m.facebook.com
[Tue Nov 17 17:17:24 2015] [error] [client 87.219.212.93] File does not exist: /var/www/vhost/stylopiel.es/home/errors/500.html, referer: http://m.facebook.com

¿Sabéis como puedo eliminar estos errores?

PLUGINS INSTALADOS
Akismet
Asesor de Cookies
BackupBuddy
Bloom
Elegant Themes Updater
Jetpack por WordPress.com
Mailrelay
Maintenance
Plugin Security Scanner
Regenerate Thumbnails
TinyMCE Advanced
WangGuard
Woo Weight Shipping
WooCommerce
WooCommerce Bulk Stock Management
WooCommerce Pay for Payment
WooCommerce Servired/RedSys Spain Gateway
WooCommerce Shortcodes
WooThemes Helper
Wordfence Security
WP-Optimize
Yoast SEO

Todos actualizados. SÉ QUE ES MUCHA INFORMACIÓN, PERO ECHARME UN CABLE.