Reply To: ERROR 500 – INTERNAL SERVER ERROR

I have same error.

The only data about that on the log is:
[Wed Oct 22 13:57:29 2014] [error] [client 31.222.112.49] Premature end of script headers: <strong>subscribe.php</strong>, referer: http://domain.net/

The url causing that is:

http://domain.net/wp-content/plugins/newsletter/do/subscribe.php

System is CentOS 6.5 with CWP Centos webpanel, wich manages too the configuration (uses suPHP).

I have no problem like that with other plugins.
Any suggestion?