Hi Ikari,
the white Pagebackground in PDF/X-1a or 3a is normal because this versions flatten all transparancies. Take an PDF/X-1a from Indesign or an other DTP-Software and you will find this Background too.

The only way to be safe in output for print in XDP v15 is to transform fonts to objects. An other workaround I found is the way to mark the font who makes trouble and change i.E. the weight and then change back. Then export to PDF and voilá it works.

But bug is bug. So pls Xara correct it.