Date: 14 Feb 2010
There seems to be an authentication issue with the Su.pr API. Even with the right credentials, a 203 Authentication error is returned.
I don't know if this is a widespread issue or just a few people, but I have emailed the service provider to clarify.
In the meantime, you can clear your authentication info (i.e. username/key) and it will still work... However, generated URLs will not be tied to your su.pr account.