Server error on checkout

I am getting an Apache error (unspecified) when I try to checkout as a logged in user. It works when not logged in. I am wondering is all my software levels are right, but I can't find any system requirements anywhere in the documentation. Could you confirm what they should be? I am on Joomla 1.7.4, mysql 5.1.8 and PHP 5.3.58.

HikaShop is compatible with all the versions of Joomla since Joomla
1.5, PHP since PHP 4 and MySQL since MySQL 4.1.