INTRODUCTION
I had this error when I tried to publish an InfoPath template to Sharepoint or when I tried to fill in an InfoPath form and submit it.
RESOLUTION
I resolved it by making the content DB offline, DELETE the whole web-application, RE-CREATE it and re-attach the content DB.
I think it has something to do with the corrupted Sharepoint IIS website. Somehow office client can’t recognize it as a valid Sharepoint website although if I go to the browser and type in the URL manually (eg. http://intranet), it renders as a proper Sharepoint site without any error.
Hope this helps,
Tommy