Magento 1.9 Internal Server Error 500 after backup
Using the Magento backup tool from within magento 1.9 causes my store to show an internal server error.
To reset this I had to change the permissions of the index.php file in the root dir to 644 from 666. It is a known issue.
There is an alternative is to upload a php file called magento cleanup more info here and more general info here