have you actually removed the impeller's cover to check that it is physically free to move?
See if the machine drains without the filter in place.
I too have some brown gunk (although not water) in a tank on the side of my dishwasher. there is a loop that goes from the tub to the drain and there is then an eccess if you wish that fills up, this is where the gunk is, and i can't seem to shift it.
Alexander S-K
July 2009