provanandmakin

We are trying to a export a drawing to a .dwg file but towards the end of the process are receiving the following message: "OpenDWG Library Error 3: The file could not be opened. Make sure it is not already open in another application." The export has then failed. What does this mean? What can we do? We have tried purging the drawing and removing symbols in case they are in some way corrupt, but to no avail. Any ideas?