I took a look at the map. Very strange. The leak check does indeed point straight through a solid brush. I loaded the map up and looked very carefully, and there was actually a hole in the wall there where the leak pointed to. It must be a bug in the compiler. I noticed that your walls were paper-thin, though, and that particular wall was made up of 3 thin brushes stuck together. I didn't have the textures, so I couldn't tell if the 3 brushes were necessary for texture alignment, but I don't think they are. I replaced them with a single, thicker brush, and it fixed the error. Unfortunately I messed some other stuff up trying to figure out where the leak was, so you probably don't want my modified map, but it should only take a few seconds to fix.
You had something like this going on:
leak
v
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What you want to do is create something like this
use clipping plane for corner
v
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It's a lot cleaner, easier to manage, and the compiler is less likely to screw up.