I have had exactly the same problem today. I have never had it before although I have exported using the same parameters on a regular basis. I am exporting from a 1920 x 1080 x 25 timeline to half res PAL wmv files.
I assumed that something on the timeline was corrupted, so I switched off the timelines being exported one by one to isolate the problem. It turned out to be on Audio one. I checked the audio clips to try and find some error but discovered nothing. Eventually I moved all the clips on Audio one to another audio timeline and the problem was solved.
It doesn't make any sense, but it worked for me. Sorry I cannot offer a more rational solution, I hope this helps?