Can’t Connect Jetpack

It looks like there is a misbehaving wp-config.php, plugin, or theme on your site.

If you go to http://www.elementoacqua.it/xmlrpc.php , you’ll see a message saying “XML-RPC server accepts POST requests only.” That’s the correct message. The problem is that there is whitespace above the message. This extra content is annoyingly important. It suggests that some plugin, theme, or wp-config.php file in your WordPress has extra “whitespace” (new lines, spaces, tabs, …) before the first “<?php” in the file.

This problem is another symptom of a more common problem: the “Headers already sent” problem:
http://codex.wordpress.org/FAQ_Troubleshooting#How_do_I_solve_the_Headers_already_sent_warning_problem.3F

Can you check your wp-config.php, theme files, and plugins for such whitespace?

You can also try to deactivate all plugins on your site, and see if that gets rid of the extra whitespace. Then, activate your plugins one at a time to determine which one is responsible.

If that doesn’t help, you can rule out a theme conflict by temporarily changing your site’s theme to a default theme (such as Twenty Sixteen) and trying again.

You’ll need to fix these issues before to be able to use Jetpack or other plugins and apps using the XML-RPC file to communicate with your site.

I’d recommend getting in touch with the plugin authors to let them know about the issue; they might already have a fix in the works for you!