The Internal Server Error is commonly caused by incorrect permissions of some files/directories on your account or un-supported directives in the .htaccess file.

If you need to change the permissions of your files or folders you can do this from cPanel -> File Manager. After selecting the file or folder you can change the permission from the menu on the left top corner of your screen. You can check exactly which files/folders have incorrect permissions in cPanel->Error Log.

For more information about this issue, please refer to:

http://kb.siteground.com/article/internal_server_error_500.html

In case the above does not solve the problem you can check your .htaccess file for un-supported directives. Usually, PHP directives are placed in the .htaccess file and this is not allowed on most servers. The .htaccess file is located in the directory where your PHPList is installed. You can edit it by using the build-in editor of the File Manager tool in the cPanel.

You can complete this by logging into your cPanel and selecting the File Manager. In order to be able to browse through the directories, you'll have to use the yellow folder icons next to "public_html" for example. Then check the .htaccess file and click the Edit button. In the newly-opened window delete all lines beginning with "php_value". Then click on the Save Changes button and check if your website is fully functional.