500 Internal Server Error when checking out after installing Attribute Stock addon

[Cdkl]

I'm not sure what I did wrong, but somehow the checkout link stopped working. The link worked before (I actually tried and purchase something, and it works), but after I installed a new attribute code (called Attribute Separate Stock: http://www.zen-cart.com/index.php?ma...roducts_id=326) the checkout button no longer works. Whenever I click on it, I get this error:

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, [email protected] and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Have anyone else encountered this problem? Is there a way I can fix this?
Any help would be great.
My website is: http://cakdel.com/