From what I saw: In Spain, inverters are not allowed to provide voltage control, and what we saw in Spain, was a voltage spike that caused generators to drop offline, which then caused frequency issues.

See report and first comment: https://news.ycombinator.com/item?id=44358668

It looked to me that regulators wanted to make solar the scapegoat for political reasons.

The report indicates to me that different operators were using a random monkey theory to make changes until the grid stabilised (they clearly didn't have a handle on the root cause of the instabilities). The regulator screwed up: they are supposed to engineer the network so it can be stable (even in the face of political pressure).