NROL-66, riding on a Minotaur from Vandenberg, got within 16 minutes of launch when an unresolvable ground problem (no better description is available at this time) caused the scrub.
The launch team twice came up with a new launch time as the issue was being worked. The second launch time would have put the launch about an hour and 45 minutes after the originally schedule. Probably that’s about the time their launch window closed.
Knowing how things work, the OSC people at the pad may have had the anomaly squared away but the corporate OSC engineers (probably remoting-in from somewhere else) didn’t have enough time to come to feel comfortable in closing out the event. The good news is that the anomaly was described as ground equipment and isn’t flight hardware.
They’ll aim for a Sunday launch now.