I have same error.
The only data about that on the log is:
[Wed Oct 22 13:57:29 2014] [error] [client 126.96.36.199] Premature end of script headers: <strong>subscribe.php</strong>, referer: http://domain.net/
The url causing that is:
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.