Sorry I respond so late, had another project coming up what must be done.
My sourch isn't updated, because all suggestions didn't work!
But maybe it comes because I start with an empty drawing where I clone a drawing with the WBlockCloneObject command
I've another thread for that piece of code -> see http://forums.autodesk.com/t5/NET/WblockCloneObjec
Maybe they are related to each other???