I had the same problem with Microsoft Live Write.
To solve it it moved all my postings to the Trash and verified that Live Writer could fetch all postings (none) from the blog without errors. After that I restored one posting at a time and verified that Live Writer could still fetch all postings (if you have many postings you can try restoring more than one between each try). When I restored the LAST posting Live Writer stopped working (Why is it allways the last?). I rewrote the posting and now Live Writer works without any problems.