Google returns a lot of results on this error, for example:
http://www.sharepointdev.net/sharepoint--infopath-forms-services/infopath-people-picker--user-not-found-error-33503.shtml
http://social.msdn.microsoft.com/Forums/en/sharepointdevelopment/thread/3e314c3e-4444-4ffb-ba78-ce6e680abeca
http://stackoverflow.com/questions/219382/profile-user-db-error-in-webpart
Also consider posting this question on Clay's blog post.
Good luck!