See this help page
You get a session_start error and it references a si-contact-form.php file
The error is similar to this (the line numbers and directory may even be slightly different)
Warning: session_start() [function.session-start]: open(/tmp/sess_aa3rmj2iquntm19df3d2higua3, O_RDWR) failed: No such file or directory (2) in /wp-content/plugins/si-contact-form/si-contact-form.php on line 1745
Or the error is similar to this (the line numbers and directory may even be slightly different)
Warning: session_start() [function.session-start]: open(/tmp/sess_aaa16408141656d9755cf49548472883, O_RDWR) failed: Permission denied (13) in /wp-content/plugins/si-contact-form/si-contact-form.php on line 1745
Also, it may or may not be followed by these two other errors:
Warning: session_start() [function.session-start]: Cannot send session cookie – headers already sent
Warning: session_start() [function.session-start]: Cannot send session cache limiter – headers already sent
The first error is the cause, the other two are effects of the first error. There is nothing wrong with the plugin code. This plugin is just asking the PHP server to open a session to set a cookie. The problem is that PHP sessions are currently not working on your server and the cookie cannot be set. The PHP server is not allowing the session to start and the error is how you find out.
The error is telling you that it cannot open the required session temporary folder because the folder does not exist or that the permissions or ownership is incorrect. This is purely a server misconfiguration, not a program error with the plugin.
Make a trouble ticket with your web host. Ask them to fix the broken PHP session feature of the web server. It will better help them resolve the problem if you include the error message and a URL to your page with the error on it. They can create the proper temporary folder needed for PHP sessions. If the folder is already there, they can check and make the necessary adjustments to the folder permissions or group ownership. Then the sessions will work as they should.