I've got a site that uses a shared SSL connection, so I thought I'd look to see what's different between the two installations. I turned off javascript in my browser, clicked the Register button in your header and then clicked the Submit button from your create_account page.
Your host is issuing a 302 redirect after the post; I think but do not know that this is the source of your problem. There are a bunch of people on webmasterworld.com that are very knowledgeable in this stuff (as are some of the people on this forum).
I'm using the "Live HTTP Headers" add-on for FireFox to view these messages.
Code:
https://server309.webhostingpad.com/~comm6179/index.php?main_page=create_account&zenid=<value 1>
POST /~comm6179/index.php?main_page=create_account&zenid=<value 1>HTTP/1.1
Host: server309.webhostingpad.com
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:14.0) Gecko/20100101 Firefox/14.0.1
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive
Referer: https://server309.webhostingpad.com/~comm6179/index.php?main_page=login&zenid=<value 2>
Cookie: __utma=218166299.1727778918.1345318472.1345322284.1345463626.3; __utmc=218166299; __utmz=218166299.1345463626.3.3.utmcsr=yourwebsite.com|utmccn=(referral)|utmcmd=referral|utmcct=/
Content-Type: application/x-www-form-urlencoded
Content-Length: 271
securityToken=<value 3>&action=process&email_pref_html=email_format&firstname=&lastname=&street_address=&suburb=&city=&zone_id=&state=%C2%A0&postcode=&zone_country_id=223&telephone=&email_address=&password=&confirmation=&email_format=TEXT&x=42&y=14
HTTP/1.1 302 Moved Temporarily
Date: Mon, 20 Aug 2012 17:42:06 GMT
Server: Apache
X-Powered-By: PHP/5.3.14
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Set-Cookie: zenid=<value 4>; path=/~comm6179; domain=yourwebsite.com; HttpOnly
Location: http://yourwebsite.com/index.php?main_page=time_out&zenid=<value 4>
Content-Length: 0
Connection: close
Content-Type: text/html
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