The most common cause of a “500 Internal Server” error is the permission level of a website file. If you open up File Manager and look in the folder where PHPlist is installed, you will notice that each file is assigned a three-digit number. These numbers represent the permission level of the file. The higher the number, the more permissions granted. However, very high permission levels are security risks. HostPapa servers will not use files if the permission levels are set too high.
If you see files or folders with permission levels set to 777, they need to be changed to 755. Likewise, if you see files or folders with permission levels set to 666, they need to be changed to 644.
To change a file or folder’s permission, click the item to select it and then click the “Change Permissions” icon (the one with the key) in the row across the top of the page. In the pop-up window, uncheck boxes until you see the total at the bottom of the window equal the required permission level.
You will need to check every folder and file for PHPlist to ensure the permissions are correct.
If that does not help, here is a link to a troubleshooting article on the PHPlist website: http://docs.phplist.com/PhplistTroubleshooting