BUG? Audio mixdown WAV empty on Cubase 5 64bit only

I seem to be having problems producing Audio mix downs in Cubase 5 64 bit.
I have read other threads about this so know that I am not alone. However, none of th etreads I looked at give a solution.

I have Cubase 5 32 bit and Cubase 5 64 bit installed.

I start Cubase 32 bit and load my project.
Cubase 5.5.2 Build 637 - Built on Sep 17 2010
Export takes about 1.5 minutes with a good playable wav file.

I start Cubase 64 bit and load the same project.
Cubase 5.5.2 Build 637 - Built on Sep 17 2010
Export takes about 5!!! MINUTES, creating an EMPTY wav file.

Incidentally, on this occasion, I closed the project, then clicked the red cross to close Cubase 5 64bit and it took 10 minutes to close. I do have that occcasionally and there does not seem to be any reason for it (nothing in the log file).
Otherwise, Cubase 5 64bit behaves well and I have a 85 track project that I HAVE to load in the 64 bit version for performance reasons. At present I develop in 64bit version and for audio mix downs I go to Cubase 32bit.

Any one any similar issues?
Cubase64bit.jpg
Cubase32bit.jpg

I am back on Cubase 5 32-bit - no problems there!

Confirmed Fixed in 5.5.3.