After you have emptied all of the water from the pump trap - put unit on drain/spin cycle without adding any water and observe operation - does it spin up to high speed during this cycle ?

While you are in there, remove the stupid little catcher cup that's in the tub-to-pump hose and ditch it, only causes headaches (you might find it full of goodies causing your issue) Looks like this:

Source: Frigidaire Gallery GLTF2940F washer - incomplete draining and E21 error code