Error On Export Options

[Beth Hanson]

I've had a recent, but frequent problem pop up; I've made some instrumental/midi files for my school choir, and when I do export/mixdown to mp3, I get "Error on Export" message. Nothing else. Can't find out what the problem is; I tried exporting as a mid file, then reimporting into a new project, which has been hit and miss. I actually tested a file I was working on last night, everything seemed to be ok, so I kept on working, then when i went to export complete file, I got the message. AARGGHH. What can I do to fix this?
It didn't do this for nearly 2 years; this is a recent ]'peculiarity'.

Cubase SX3 0n 10.4.8 Power PC G4, 1.4 GHZ processor, 768 RAM...