Check your file permissions to make sure nothing is set to a CHMOD of 777. Due to security reasons, 777 File Permissions cannot be used. For Files, set permissions of 755, and for Folders set permissions of 644. You can check and change these using an FTP client, or through cPanel's File Manager
Fixed, I commented the section repaired in your .htaccess. Most CMS systems have a commented line that says "RewriteBase /"; yours didn't, so I added it. That appears to have solved the problem.