This means the form is failing the server validation.
My guess is that the checkbox groups are throwing it off, but I'd want to look at the code to verify.
Can you find the server validation code on the page and copy and paste it into a reply to this thread? If not maybe attatch the page, but really I just want to look at the server validation portion.
You could narrow it down more by adding <?php var_dump($_SESSION); ?>
This would give you more information about which field(s) are actually failing validation. I could probably tell by looking at the code directly, but the var_dump would tell me where exactly to look.