Hi Alex, the inner surface buckles up and folds back over itself in this area here:
That kind of self intersecting surface can tend to cause problems with booleans.
Another thing I noticed is that the edges are just very slightly misaligned:
Those are about 0.005 units apart from each other - that's kind of a sensitive distance range because it's small but yet just outside the tolerance value for where things are considered coincident and merged together. I'd probably try to true these up and get them more precisely aligned if possible.
- Michael
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