wp-cron.php returns "Connection closed" once per hour

I have three sites on the same VM. All three run cron through command-line (PHP-CLI) every 5 minutes. One of those is always silent while the other two return “Connection closed” once per hour. One returns that failure at 10 minutes past the hour and the other – at 40 minutes past the hour. So every hour I receive two emails with the content of “Connection closed”. There is nothing else.

Can someone shed some light on this?