Ok - it's likely that you don't have enough free space on your drive to perform the final action. If this happens again, don't push the "OK" button... instead: go to the folder that you are exporting the file to. In this folder should be a file with the same name as your exported video, but with the file extension ".temp". Rename this file to something else, for example, "My Movie.mov" - just so long as it doesn't have the same name as the file you are exporting. Then push "OK" in MixEmergency. The file that you have renamed will contain your exported mix.