ThomasKlein

@aizar
restarted nginx: service nginx restart
flushed memcache server: echo 'flush_all' | nc localhost 11211
Problem still exists.

@petermolnar

nginx/error.log

2014/12/16 16:47:16 [error] 19502#0: *45 FastCGI sent in stderr: "PHP message: PHP Warning:  wp-ffpc not clearing unidentified post  in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 126" while reading response header from upstream, client: 80.187.100.33, server: _, request: "POST /wp-admin/options-general.php?page=wp-ffpc-settings&flushed=true HTTP/1.1", upstream: "fastcgi://unix:/var/run/php5-fpm.sock:", host: "108.61.210.226", referrer: "https://108.61.210.226/wp-admin/options-general.php?page=wp-ffpc-settings&flushed=true"

2014/12/16 16:49:40 [error] 19502#0: *67 FastCGI sent in stderr: "PHP message: PHP Warning:  wp-ffpc not clearing unidentified post  in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 126" while reading response header from upstream, client: 80.187.100.33, server: _, request: "POST /wp-admin/options-general.php?page=wp-ffpc-settings&precached=true HTTP/1.1", upstream: "fastcgi://unix:/var/run/php5-fpm.sock:", host: "108.61.210.226", referrer: "https://108.61.210.226/wp-admin/options-general.php?page=wp-ffpc-settings&precached=true"

Precache log

permalink	                           generation time (s) size ( kbyte )
https://108.61.210.226/hello-world/	                0.04	       10.97
https://108.61.210.226/sample-page/	                0.04	        9.92
https://108.61.210.226/category/uncategorized/           0.02	        7.51
PHP Warning: file_get_contents(https://108.61.210.226/uncategorized/): failed to open stream: HTTP request failed! HTTP/1.1 404 Not Found
in /tmp/wp-ffpc-precache.php on line 14
https://108.61.210.226/uncategorized/	                0.02	           0

WordPress with WP_DEBUG

[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc init starting in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc 127.0.0.1:11211 added in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc checking server statuses in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc 127.0.0.1:11211 server is up & running in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc checking server statuses in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc 127.0.0.1:11211 server is up & running in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc original key configuration: $scheme://$host$request_uri in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc setting key to: meta-https://108.61.210.226/hello-world/ in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc get meta-https://108.61.210.226/hello-world/ in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc failed to get entry: meta-https://108.61.210.226/hello-world/ in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Warning:  mysqli_real_connect(): Headers and client library minor version mismatch. Headers:50540 Library:100015 in /var/www/html/wp-includes/wp-db.php on line $
[16-Dec-2014 17:28:13 UTC] PHP Notice:  PluginUtils  – getting option wp-ffpc in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc init starting in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc 127.0.0.1:11211 added in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc checking server statuses in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc 127.0.0.1:11211 server is up & running in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc clearing scheduled hook wp-ffpc-precache-task in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc original key configuration: $scheme://$host$request_uri in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc setting key to: meta-https://108.61.210.226/hello-world/ in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line 131
[16-Dec-2014 17:28:13 UTC] PHP Notice:  wp-ffpc set meta-https://108.61.210.226/hello-world/ expiration time: 300 in /var/www/html/wp-content/plugins/wp-ffpc/wp-common/plugin_utils.php on line$

Thank you for your help, it was a formatting error.

The problem still exists.