Similar problem, although I believe that being on a shared SSL might be triggering the problem (I cannot be certain). SEF urls don't work. With SEF urls turned off and .htaccess still enabled (unchanged) I get a SERVER NOT FOUND error. diannajulia.com is the current site. Note the sub domain when you go into checkout.

Well its sorted out now. I played with a few things but I think this is what secured the win:

1. Turning off rewrites in my .htaccess:
#my secure domain is securesite-1.ssllock.net/diannajulia.com/
RewriteCond %{HTTP_HOST} secure-1\.ssllock\.net\.diannajulia\.com
RewriteRule .* - [L]

2. Set SEF CHECKOUT URLS to no in components>Joomsef > Manage Extensions > Joomla

3. Purge urls
4. Clear Cache
5. Update URLS

Still having major dramas. Virtuemart works fine because its used in checkout - all data is sent via $_POST. But menu items fail.

Do you have a .htaccess rule I could use for this shared ssl domain?

securesite-1.ssllock.net/diannajulia.com