Hello!
When activating any Vienna Suite 64-bit plugin in Nuendo 5.5.3, audio is muted and there's no other way to get it back than restarting Nuendo. This happens randomly, many times a day and a lot of time is wasted when all samples, etc. need to be loaded again. Highly annoying too, during creative bursts, like just a minute ago.
I posted about this sometime ago in Nuendo forum and Dietz told me that Vienna Suite team has been informed. Have you found out how to replicate the problem and is there someway around it?