I don't see any reason for the flow stoppage except a clog in the vertical pipe. Is is possible that the leaves, sticks, or some other crap got sucked into the drain and clogged the pipe, likely near the bottom? If you know the exact height of the stand pipe, jam a 2 x 4 x (the same length + 12" to hold it) down the stand pipe and make sure you hit the horizontal PVC at the bottom.
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...or have I got the physics wrong?
