Ok,- three more notes. I was in a VR project, and this only crashes with a Development Packaged build. It works fine in the editor, and in shipping it skips the asserts.
I identified the main culprit- setting the object that does the Scenecapture2D->Capture() to the physics update tick. Somehow we had bumped that. When I put it back to pre-physics it works.
Thanks!