Many users encounter some weird magento internal server errros (Error type 500). I will try to list the most common solutions. These errors are not always caused by the same reason. You should try to take a look at your server’s error logs to get some help about this error.
You can get additional info about the errors by Turning on Developer Mode. Look in the Magento bootstrap file (index.php), you’ll see lines similar to the following
Uncomment these. In a production system, you’d never want to have your errors display to the browser, but while developing having an errors and warnings thrown immediately in your face is invaluable. This way, you will see the actually problem which lead to the Internal Error Server. In almost cases, the reason is that there is an exception throw after output is sent to browser.