F18 is a draining problem. And as you say you carnt open the filter suggest something is jamming the filter up. For this tip the machine 45 degs backwards against a wall. Then take off the plastic trim at the bottom. Then undo the filter unit from the machine and let the pump fall down inside machine.
Then take off the black hose which goes from the tub to the pump and clear anything thats coming through there.
Then the filter will undo and you can clear out any blockages from there. Then refit it and see how it goes from there.