If you so happen to have a DigitalOcean droplet (if you don’t, you should!) and are seeing this <sarcasm>helpful</sarcasm> little error message below…
Error Details: The Jetpack server was unable to communicate with your site [HTTP 500]. Ask your web host if they allow connections from WordPress.com. If you need further assistance, contact Jetpack Support: http://jetpack.com/support/
…then check your apache error.log in /var/log/apache2. You might see a bunch of lines that contain something like
PHP Fatal error: Uncaught Error: Call to undefined function xml_parser_create() in /var/www/html/wp-includes/class-IXR.php. If that’s the case, then do an
apt-get install php-xml-parser, restart apache with
systemctl restart apache2 and try again.
Then all should be well.
I found this solution on the DigitalOcean community forums.