How to Solve Magento 500 Internal Server Errors. Many users will encounter some strange Magento 500 internal server errors (error type 500). I will try to list the most common solution. These errors are not always caused by the same reason. You should try to look at your server’s error log to get some help on this error.
You can get additional information about the error by turning developer mode. Look in the Magento boot file (the index.php), you will see a line like
#Mage :: SetIsDeveloperMode (true);
#ini_set (‘the display_errors’, 1);
Uncomment these. In production systems, you never want to show your mistakes browser, but there was an error and warnings immediately thrown in your face is priceless in development. As a result, you’ll see the actual problem, which led to an internal server error. Almost in the case, the reason is that there is an exception thrown output after being sent to the browser.

Solution # 1

This error may be caused because you do not set the correct permissions for the Magento directory. To solve this problem go to the File Manager, and then change the index.php file permissions from 664-644 also change the permissions to download / index.php file 644, and otherwise, when you try to access the System> Magento connection> Magento The Connection Manager to log on to the Magento admin (Magento-installation), you will get 500 internal server error.
You can also try this tool, it is a Magento’s removal tool. It is your complete Magento install the correct permissions:
Download it
Unzip the -cleanup.php Magento Magento installation to your root directory
Browse to http://yourdomain.com/magento/magento-cleanup.php

Solution # 2

Your server does not support some of Magento specifications. You can easily use Magento inspection tool for testing. Follow these steps to check the server software, if there are not compatible utility will show it.
Download it
Unzip the -check.php Magento Magento installation to your root directory
Browse to http://yourdomain.com/magento/magento-check.php

Solution # 3

If you receive a specific page of your website weird 500 internal server error, which may be a problem of resources. I got some of the products page and return http://yourdomain.com/checkout/onepage internal server error. I found my Magento installation .htacess files are somehow reset, set the value php_value memory_limit to 32M, as long as I propose, the internal server error disappeared! You should use at least more than 256M 600-700 single product. Magento is very resource intensive, it is easy to get these kind of mistakes, if you’re trying to save some money from hosting.

Solution # 4

It is located in Magento .htaccess file in the root folder. This would be in this case, if you encounter on every page internal server error. Try to remove for testing purposes
If the file to your site running for a long time, then it must be a change in the server, submit only one vote hosting company.

Solution # 5 (reviews from Huberto)

If the curl extension is missing, you can get 500 internal server error. You can use the following command to install it:
Apt-get install curl libcurl3 libcurl3-dev in PHP5 curl