Also very effective way to avoid this is to export pdf to older version. Some older RIP's can't handle the transparency right, specially with spot colors. Try version 1.3, that should work as it flattens the transparency (with converting spot color to process colors as I said earlier). I'm quite sure that this problem is caused by the use of extra color channel and transparency in pdf. And the printer's software can't handle them right. Sometimes there can be color shifts if there is RGB image with transparency over CMYK elements, if the color management isn't done correctly, but the white box is really rare.
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