There is just a problem with the normals. What I did (in my modeller):

resetting the normals to standard
inserting some "edges" into your cubes

( download: http://adp.pictureparty.de/test/Poser_test.obj )

Reason: Poser has some trouble with plane surfaces, because Poser is optimized for so called "organic objects". Creating "soft edges" in a Poser model makes far less trouble and looks nices.

Edit:

Just download and save the file into your character folder.

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