Problem was outdoor module battery was poor (however, was reported at 67% by netatmo). If data is missing from netatmo the netatmo module will stop updating and keep the old temperatures. If MM is restarted a spinning wheel will be shown. Hope that the new batteries did the trick.
A New Chapter for MagicMirror: The Community Takes the Lead
Read the statement by Michael Teeuw here.
Read the statement by Michael Teeuw here.