Contact form only works once

I just switched to GoDaddy using a Linux server and I am trying to get the existing site functioning again using the Contact form solution pack.

I'd like to add some validation as well, but at this point I am trying to just get the form working.
contactus2.php

When I fill out the form, I do not go to the thankyou.php page. I get an error:
Warning: Cannot modify header information - headers already sent by (output started at /home/content/83/9744683/html/contactus2.php:13) in /home/content/83/9744683/html/contactus2.php on line 21

I've tried removing the redirect to thankyou.php and the form will send the email, but it has only worked once. I tried it again and it doesn't work. I'm not really sure what the issue is, or if it's something with GoDaddy.

I've used the contact form solution pack on other sites and I've never had this issue before.