I don't know what HeadMeta.php is or what it does, but if you get the above error then it seems that cforms is conflicting with another plugin that also utilizes PHP session management.
I did not get the above error message, however, I noticed that the captcha isn't working on your site (could be related to broken PHP session management).
I also noticed that you've got a JS error in valforms.js – check applyOnSubmitToForms()…
Try turning off your other plugins one by one to see which is the actual culprit and take it from there.