I just had the same problem last night — and found a solution. Maybe and hopefully it works for you also: The problem was caused by changing the external hard drive with the render files to another drive. This happens accidentally, because I started FCP with the external drive switched off. Since then FCP writes everything on the internal drive instead of the correct one. So when I start the down mix, the program missed some files and crashed.
What I did — and it solved the problem — was deleting all the rendering files with the render manager and switch the hart disk in the fcps system settings back to the external drive. After a long time of rendering again – but on the correct hard drive – I start the mix down and export again — and it works perfect.
The only thing I didn’t find a solution for is that some files of the last thing I did (voice over) are now on the internal (wrong) drive. I copied them to the correct drive but don’t know how to tell FCP to link to this files. But as everything works for the moment, I am quite happy.