I spotted this thread earlier today after encountering the same issue with Xpress 8.12 under WinXP SP3, so it has most definitely *NOT* been resolved in the latest update.
I should explain (in case anyone from Quark is listening), that I am not an inexperienced tyro, having used QX consistently for professional design and print since version 3.0 on both Macs and PC.
Having successfully used the transparency feature in Xpress on several jobs until today, it seems to me that what makes it fall over is the number of elements on which it is used, and/or the size of any background images to which such effects are applied.We had placed several coloured pic boxes on top of an 80MB cmyk background eps and then placed text boxes over those with drop shadow applied to the text. Quark did not like it one bit and refused to print the job or export to PDF.
After much faffing around with no worthwhile improvement, I eventually saved the dps as two separate EPS's, brought them into Photoshop and re-created all the fancy stuff in PS, before saving out as a flattened EPS and bringing that back into the Xpress document. As you might guess: problem solved. Which kind of defeats the object of providing tools one cannot properly use, doesn't it?
It is not impossible that one or more xtensions compound this issue (or perhaps even cause it?), but without turning them all off and going through the laborious task of re-enabling them one by one after trying to print, there is no way to know. If someone has the leisure to do that I'd be interested to hear the results. Xtensions have always cause grief in Quark as all long-term users will know only too well. It would be strange if they were now all magically well-behaved. 
The lesson I've learned is that, whilst QX 8.12 may save some time in allowing the user to create effects that would otherwise require several roundtrips into Photoshop, unless these effects are kept to a minimum it really is no more productive a page layout application than version 4.11 was - which we still use BTW!
Maybe it will all get fixed in version 10?!!
p.s. Although the forum does work with Firefox typing is a bit hit and miss. But as I am not about to start using any Microsoft browser I can live with that!