> much of the available energy was being used to pump water from low lying river basins into reservoirs – the only practical way to store energy on a large scale. However, this capacity has a limit and, with the reservoirs almost full, it cannot continue to be stored indefinitely.
There has been unusually heavy rain in Spain for an unusual length of time now, and reservoirs are well beyond average capacity.[1]
I wonder if this anomalous situation has anything to do with the blackout. Maybe the inability to continue pumping water led to the shutdown of the solar, maybe for economic reasons as is suggested in some other comment?