Was going to chime in as this was actually the same problem I had today. Was getting sand in the pool after replacing XL valve and it working great for 12 hours. Went to backwash and really had no backwash pressure and really no rinse AND it was leaking out the waste port.

Thought is was the spider gasket (no), then a broken lateral (no) but like you discovered my pipe snapped at the lateral assembly. Replaced and back in business for now.