Please help. Suddenly low PV wattage into EP 500 from 4 PV350s

There is suddenly only approx. 25% of watts coming from my four PV350 panels (2S2P) into my EP500 unit. Nothing has changed. But with full sunlight up until a week ago it used to bring in approx 1000W. Now it is not going beyond 250W. The voltage coming from the panels is 81V so that seems about right.

Any ideas? Thank you.
Marc

So I found that two of the four PV350 panels have stopped producing any voltage. What can be the reason for that? How do I fix it?