Have had my current Polaris 380 for around 6 years. Nothing has changed...but suddenly the cast to my backup valve split 3/4ths of the way around above the threads. Replaced with backup valve from previous 380, which split within 3 weeks. What gives? No problems for 13 years, now two split valve cases in a month.

My polaris 380 is stuck in backup mode. Fliter screen is fine and wheels do turn.

It sounds like you have a clogged drive turban jet or an internal hose has popped loose. I normally tell people to double check the backup valve to verify that it has been installed the right way (widest part toward the wall,)
There is a total of 6 jets, three vacuum jets blow debris into bag, one thrust jet bows water out of the nozzle on back of unit, one drive jet that operates the drive gears and one operates the sweep tail.
It is best to disassemble down to the "water management system" listed above and rinse well, shaking it in all directions. Any small debris can clog the jets, including ants!

I think it is a coincidence. On the other hand, there could be a restriction downstream causing the pressure to be too high. I would check for this but either way you will need to buy a new one. I hope this helps.

Exactly: No more than 3' from the head.
And that first section of hose is the "Soft" Clear style. The Back up Valve will be installed on it... And that hose should be no longer than the deepest part of the pool.
See pic, back up valve inside the square outline...

One hose length longer than the longest run

The backup valve should shut off completely. All the pressure should be going to the Polaris for 3 minutes, and then all the pressure to the backup valve for 35 seconds. If water is coming out of the backup valve when it should be off, then needed pressure to the Polaris is being wasted.

If the Polaris is working fine, then no reason to fool with it. If not, you need to fix the backup unit. You can open it up and clean out debris and readjust it. (You'll really need a "strap wrench" from a hardware store to get off the outer ring. I paid $6). You can purchase the the inside mechanism if you can't fix it. You can also purchase an entire backup valve...outer case and inner mechanism. I bought one for $42 off ebay but they sell for $50-60 and my local pool store quoted $100.

One often overlooked area is loss of pressure through worn swivels.This can cause unexplained sluggishness in performance. Most importantly is to make sure that there is a screen in feed line and that it is clean.