Problem with Nuendo 4,,,, anybody?

I tried to export 4 mono guitar tracks, grouped, as a stereo audio file. The result was a file with no sound whatsoever. I've done this a million times before and never had this problem. What's happening? The group track is stereo and was, as is normal, soloed for the bounce. It produced a wave file with no wave. I'm baffled. I also tried to bounce them to the stereo outputs. no joy. Anyone got any ideas?
Thank you,
D'oa