Pump failure
I just fixed the same problem on my DE-500P. The pump filter was blocked and there was a layer of scummy lint-like stuff in the bottom of the pump reservoir (not the main - or overflow tank) also blocking flow down through the filter to the pump. Repair steps are as follows:
1. Remove the overflow water tank and the two side covers (five screws in each). Note that as you go through the removal of all the covers and other procedures, there are two lengths of screws; don't switch them. If you lose track, the rule of thumb is use the short screws for going through a single layer of plastic into metal.
2. Remove the rear cover (take out the screws along the bottom and on the sides). It will still be attached to the front top cover at the top with tab fasteners. Gently pull out the bottom of the cover while depressing the top of it on both sides with your thumbs (about an inch and a half inside the trim ridges) to clear the tabs. Ease the top part of the cover back and remove it (it will have to tilt to one side, don't forget this when putting it back on).
3. Remove the top cover, not forgetting the two screws on top by where the back cover attached (go gently, there's a wiring harness) and turn it to the side. There was enough slack to leave the wires connected; just be careful to not damage them.
4. The filter looks like a small transparent in-line gas filter. Remove the pump filter, leaving the short tube attached to the top of it. I quickly atttached a length of tubing to drain the reservoir into a bucket. I couldn't find a replacement filter, so I filed a slot in the side of the bottom half of it to remove the gunk using the wire from a twist tie (the upper half had some tabs inside that would have been in the way). Afterwards, I wrapped it tightly with stretchy electrical tape and put a wire tie over the tape at the slot to keep the tape from being able to creep loose over time. Don't use a fuel filter, the paper element is too fine (you need a screen) and the paper will fall apart in water.
4. Remove the float and bracket from the pump reservoir. The reservoir screws are not all apparent; there's also a screw way over on the left and one coming in from the right. I used a 12 inch #2 phillips screwdriver with a magnetized tip for most of this. Remove the pump reservoir and clean out the scummy stuff in the bottom and where the tubes attach. When removing the reservoir, you'll have to detach the vent line that goes through the bulkhead to where the big tank was and pull the tab above it down slightly while easing the tank out.
5. Clean out any dirty tubes, paying particular attention to the coupling for the drain hose and the long drain hose itself.
6. Put the reservoir back in and hook up all the tubes and filter. It does make a difference in which direction the filter goes back in. You want the water from the reservoir to go through the filter from the outside of the little screen; that leaves the maximum room for crud build-up before it plugs up the screen again.
7. Depending on your confidence level, at this point you may want to plug in the dehumidfier, put the overflow tank in, turn it on, turn on the pump, and fill the pump reservoir to check for functioning and leaks. If you do this, just remember that the circuits are hot and not all are insulated.
8. If all is OK, unplug the dehumidifier and put the covers back on. Total time was about an hour.
