OK, after a very brief google search, it appears the controller reports this error when it can't "see" the sensor. I can think of only two possibilities: either the sensor is bad, or its connection is. Well, three if you count a bad mother board.
If you're handy, you could try this. FIRST, TURN OFF ALL THE BREAKERS TO YOUR PAD.
Do not work on your controller while it has power, you could fry your board,
or yourself!
All the sensors connect to the circuit board, so you'll have to undo a few screws to find the terminal where they connect. You'll have two or three sensors, they're all clearly labeled. You probably have all three. You can try two things:
1. Remove the wires for the solar sensor and inspect them. Broken or corroded? Try scraping them very clean and reconnecting them to the board. Snug up the terminal screws for a solid connection (but don't over tighten).
2. If that doesn't help, then you can try swapping the solar sensor with the air sensor. If the controller reports the same error, it could be your circuit board. If the error "moves" to the air sensor, then you've probably got a bad sensor and it needs to be replaced. They're pretty cheap, as Pentair gear goes, but you'll need to get up on the roof to replace it. Here's one place to get one: