When I squeeze the hose at the top, I can feel that something is flowing through it. Is there a pump in between? Otherwise, I would try to suck it out directly from the hose at the waste ink tank using a syringe.
edit
There is no pump up there. Toward the hose there is a small hole where you can carefully insert the cleaning sticks used for printhead cleaning. It’s best to use the green side without the fabric and gently poke around inside. After that, it should start draining again.