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.
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).
