I get these, on both my test machine (2GB) and on our live site (8GB). On the test machine the RAM is usually between 50 and 80%, but it isn’t swapping. I’m the only user even looking at it, and there are no mails going in or out (disabled), and no new posts being added. It seems to give this error regardless of if I am doing something like a backup or a restore or changing post ownership in the CLI, or the machine is just idling.
Ubuntu 16.04, on AWS, straightforward install per the cloud instructions, happens regardless of if I’ve got plugins loaded or not.