After running more diagnostics and considering all factors..
I (may have) have hit upon the solution. Up until recently, I have used liquid or gel detergents. Not long ago, my wife brought home the small multi-compartment "pods" to try. I figured "why not?" The pods are placed in the soap compartment, of course..
It turns out that the cellophane-like wrapper (which is formulated to dissolve upon contact with hot water) was not dissolving, getting caught in the flow channels and causing the error code! It only seems to happen with a brand I recently purchased. And my water temperature is sufficiently hot.
After three cycles using the gel, there have been no problems. My particular model does not appear to have a contact macerator (but it is well-filtered).
So... What is larger (and more flexible) than a small morsel becomes trapped!