[Plugin: Fast Secure Contact Form] Error when moving to PHP 5.3

I just updated my web hosting to use PHP 5.3, and now I get this message: Warning: session_start() [function.session-start]: Cannot send session cache limiter – headers already sent (output started at /home/harmonyschool/dev.harmonyschool.org/wp-content/themes/thesis_184/custom/custom_functions.php:1) in /home/harmonyschool/dev.harmonyschool.org/wp-content/plugins/si-contact-form/si-contact-form.php on line 2189.

Does anyone have any suggestions?

Thanks,
Paul

https://www.ads-software.com/extend/plugins/si-contact-form/