Any wisdom about how I can fix this?
Many months ago, I under-tightened the rear axle nuts on my commuter bike and the wheel came off while riding. It’s happened two more times since then despite my best effort to tighten. It looks like the gripping washer has actually damaged the hanger on this aluminum frame, which you can see as lines in the silver area in the photo (nut removed, green-rimmed washer pulled out from it’s normally flush position). It looks about the same on the other side of the bike.
Scott Bikes says, yep, it’s damaged and the grooves probably explain why the wheel has come off repeatedly. They say I should replace the axle and spacer hardware and just tighten it really well.