If you are self-hosting WordPress on your own Linux server, you need to make sure you have installed the PHP-XML module. Without this module, you may see an error from MarsEdit about a failure to “parse the XML document”. This error comes from your WordPress installation, and typically means it doesn’t have the library support to manage the XML-RPC protocol that MarsEdit uses to communicate with WordPress.
For example, if you use a Debian-style package manager, and are running PHP 7.2, you might install the required PHP-XML library by running this command from the server’s Unix shell:
apt-get install php7.2-xml
After installing the PHP-XML library, you will need to restarting your HTTP server before PHP is able to take advantage of the updated XML support.