This is a problem that I think I created. I posted from Live Writer to my blog, but then deleted the blog on the site. Since I did that I get this error message and can't post any more.
I think this is an error that Community Server returns when you try to publish a post that has the same title as an existing post. Is it possible that Community Server doesn't truly delete the post, but just puts it in a trash folder or something?
In any case, you might try changing the title to see if that fixes it.
I experience the same problem as TigerTheFrog. After deleting the blog directly from my blog server I was no longer able to publish the post from LiveWriter.
Changing the title did not work either. I had to copy my post content to Microsoft Word. Delete the post from the "Recently Posted" listing. Then create a new post - copying the content back in. I was able to use the same post title as before.
It would be a nice improvement if when this error is generated that the blogger is given the option to Post Again As New thus saving several steps.