In this case it *might* be that you have granted only read-only access
to the application. Can you check that? There are two modes of
access: read-only and read-write for users to grant to different
applications. I only say that this might be the case because you
appear to be able to read from the Twitter.com API without any errors,
but failing on a write.
Let me know if that is the problem or not for reference and to help
others that might have the same problem on the mailing list.