API shows problem "missed_unit" but nothing shown in Hive web app

Hi there,
one of my rigs is restarting but showing no error message in Hive web app. So I start logging API response for (/farms/id/workers/id) it every minute and everytime just before it restarts, the API for this worker states this only unusual thing… after restart the “problems” is empty.

"stats": {
    "problems": [
      "missed_unit"
    ]
  },

Is there any list of possible problems that can be mentioned there and what exactly does it mean? What does it mean “missed_unit”…
Thanks for any help!