You know, I think Yanick is onto something.
Currently, I'm using only Kontakt 3 ahead of Play in VE Pro (VST 3 plug through the server in Cubase 5) and I have no problem using Play or even loading instances of it. I noticed the errors were gone when loading Play and that this happened after I deleted the default Vienna Instrument out of VEPro. All this on Vista 64 using two machines and two VE Pro VST 3 plugs.
So it may have something to do with a conflict between these two particular instruments.