Hi Matteo!
you've installed an old version of Moodle in a modern PHP environment e.g. Moodle 2.9- running on PHP 7.0+.
In my case, it's exactly the opposite. I have instaled what seemed to be the latest version of Moodle,
3.3.1, freshly downloaded from download.moodle.org, in a server running
PHP 5.6.19. I apologize, I should have include version info in my reply.
Anyway, setting "always_populate_raw_post_data" as stated by the PHP warning caused the popup with the error to stop showing up, although I still see that error in the console occasionally.