If you get this error try to set up correct permissions for .htaccess and index.php under the directory.
For example, try setting chmod 755 index.php and that should fix it.
You can do that by accessing via SSH and changing the permissions or using FileZilla and changing the file permissions individually.
Sometimes you can’t figure exactly what the error log is, but the original message you get for this error is usually an Internal Server Error in your browser.