This issue just popped onto the radar yesterday. Revit was generating the following error message while exporting to dwg.
The issue is the use of illegal file name characters in the name of a view and/or project name. Revit has to create a file during export or plotting to a file and if the sheet name/view name or project name contains illegal characters Revit will complain and fail to do what you ask.
Just keep it LEGAL!