Solar freeze?

[JohnT]

My first guess would be similar to the above. You probably have an air bleeder at the top that was either a little sticky from the cold or had water frozen in it already

[Poconos]

Something else to be aware of is that a solar panel can freeze pretty fast. They are designed to be good solar heat absorbers, and therefore become good heat radiators. Quite possibly, looking at a cold clear sky at night, some water froze and when the pump started a chunk of ice jammed the bleed valve. However, I think the ice should have melted pretty quick and let the valve close properly. It is quite possible the panels hit freezing before the temp sensor on the freeze protecton circuit caused the pump to start. I'd just be aware this problem happened once and simply keep watch to see if it happens again.
Al