Update:
This morning we again tried backwash and waste, same result. The pump is straining, and I keep shutting off after a few seconds to avoid blowing the fuse (uhh, actually popping the breaker, which is in the basement, etc etc).

On filter, the pump starts normally in under 1/2 second.
I did try halfway between filer and waste, no luck.


I ran a complete check and TOOK NOTES!

My filter has, clockwise
Filter
Waste
Closed
Backwash
Recirculate
Rinse

The pump works ok on
Filter
Closed
Recirculate



But the good news is the pools went from pea green to sea foam green. We could see a dark clump on the bottom and got a bunch of leaves out.

Then I tried backwash one more time and it worked! I ran it until the water looked clear. Of course, nothing is connected to my fancy new french drain, so my yard is full of water again. (the drain should clear it in an hour or two)

So we are bleaching away and vacuuming the bottom.

I am starting to suspect the valve. Maybe I should replace the whole filter, it's at least 15years old.