WHMCS WP Integration: Error when trying to change billing cycle on front end

Hello, I just installed this plugin on a fresh wordpress install (not multisite). I get the following error when going through the checkout process and trying to change the billing cycle in the drop down. The drop down works fine when I visit whmcs directly and not through wordpress.
wordpress is in root and whmcs is in a subdirectory named client (site.com/client).

[Tue Apr 03 00:48:58 2012] [error] [client 216.110.94.228] Premature end of script headers: index.php, referer: http://site.com/dropdowns/sub-page-1/?whmcsportal%5Bpage%5D=http%3A%2F%2Fwww.site.com%2Fclient%2Fcart.php
[Tue Apr 03 00:48:58 2012] [error] [client 216.110.94.228] SystemException in API_Linux.cpp:172: setuid() failed: Resource temporarily unavailable, referer: http://site.com/dropdowns/sub-page-1/
[Tue Apr 03 00:48:58 2012] [error] [client 216.110.94.228] Premature end of script headers: index.php, referer: http://site.com/dropdowns/sub-page-1/
[Tue Apr 03 00:48:58 2012] [error] [client 216.110.94.228] SystemException in API_Linux.cpp:172: setuid() failed: Resource temporarily unavailable, referer: http://site.com/dropdowns/sub-page-1/
[Tue Apr 03 00:48:58 2012] [error] [client 216.110.94.228] Premature end of script headers: index.php, referer: http://site.com/dropdowns/sub-page-1/