For those who have not yet experienced this error while using Window Live Writer, hopefully this will save you some time.  Peter Laudati's post helped me resolve this one.  He explains why it happens and that it is a known issue.

The exact error reads like this: "Blog Server Error: Server Error 0 Occurred - Object reference not set to an instance of an object."

It is caused by the following steps:

1. Create a new post in Windows Live Writer.
2. Publish the post to your Community Server blog.
3. Delete the post using the Community Server control panel.
4. Re-publish the post from Windows Live Writer.

To avoid this, I would recommend that you publish your post as a draft until you are sure you want to make it live.  Again, see Peter's post for more details.