Your post has been saved as a draft instead of published. You must go to Blogger.com to publish your post. To prevent these errors in the future, request a review at: http://www.blogger.com/unlock-blog.g?blogID=NNNNNNNThis is a pretty bad choice of error codes, since your post is stored in the system, even if it isn't published, since 500 is more-or-less "Internal Server Error", which implies that whatever you did didn't work. I had a bunch of duplicate posts trying to figure this out before I started looking at the output and checking the stored drafts. I'm not sure if there really is a good code to return in this case, but 500 seems wrong.
I've filed for review, but that seems to be going nowhere slow.
Update: I'm off the hook:
Hello, Your blog has been reviewed, verified, and whitelisted so that it will no longer appear as potential spam. If you sign out of Blogger and sign back in again, you should be able to post as normal. Thanks for your patience, and we apologize for any inconvenience this has caused. Sincerely, Blogger Support