The problem occurs when an STL file is created from a cube-based piece. If the piece contains an "inside corner" and a non-zero bevel is selected, the resulting STL file is created but contains "inverted normals". Many pieces do not have inside corners and work fine, but for example a burr piece that is not notchable usually contains an inside corner where three faces meet (like the corner of a room).
This is a tricky situation, and the STL file is fine if a zero bevel is selected. I am not certain that these inside corners are the problem but every piece I've exported with an inside corner has failed, and all of them without inside corners have worked. But it seems that there should be no inverted normals.