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Is it possible you're being too aggressive with your cleaning and you're scrubbing the o-rings right out of the chain? Something to think about. They're just little rubber rings. Don't be using a wire brush, and even aggressive plastic with a hard scrubbing might encourage failure.
You might know this, but that washer is probably a locking washer. Meant to have a side bent upward against the nut to prevent it from backing out. If it's just a plain old washer, you might want to get the proper one or safety wire the nut.