Magento 1.9 – Fix 500 Internal Server Error on Checkout

Most probably, it is related to missing php extension on your server. I had similar issue and in my case it was related to missing php-mbstring extension on server.

You can debug the error by enabling display_errors in your index.php file.

ini_set('display_errors', 1);

Then on your checkout page, you can check the error output on firebug or similar debugger.

Your shop fails while doing the ajax request, presumably there is something wrong with the quote-object that causes an exception, and if this happens, you are redirected to the cart. I have debugged such errors in the past by doing this:

• turn on the Magento developer mode and error_reporting
• check that [magento]/var/log/ is writable by the webserver, sometimes this means they need permissions "777"
• I use Google Chrome, so I open the developer tools and change to the "Network" tab
• walk through the checkout; as soon as you notice the site reloading, hit "ESC" and inspect the ajax-request you now see in the "Network"-tab. It should display an error with a stacktrace.