I'm getting reports of this error message, on 3 out of 2000 ish sites on our multisite. I've installed the Heartbeat control plugin on a site and set the frequency to 60 seconds and control heartbeat locations to Allow only on post edit pages. I'm also trying disable everywhere on another sub site.
This issue started on version 4.5.4, I updated to 4.6.1 and still seeing it.
Any idea how to fix it?