Here's the most recent FAA advisory (all times UTC)
EVENT TIME: 23/0043 - 23/0200
GROUND STOP ALL UAL MAINLINE DOMESTIC DEPARTURES TO DOMESTIC
DESTINATIONS. INTERNATIONAL, HAWAII AND ALASKA DESTINATIONS ARE
EXEMPT. UAL SUB CARRIERS ARE EXEMPT. UPDATE AT 0200Z.
There is also this from the FAA:
NATIONWIDE UAL MAINLINE ONLY GROUND STOP HAS BEEN IMPLEMENTED DUE TO A
COMPANY WIDE AUTOMATION ISSUE.
I wonder what an "automation" issue is?