Problem: When trying to “Add new Item” to a SharePoint 2010 list (document library, task list, etc), and you receive this very helpful error.
Cause: Alternate Access Mappings. It displays this error message when I cannot resolve the URL.
Solution: Add the mapping. Example: You are acessing the site via: http://mysharepoint.domain.com, however you do not have an Alternate Access Mapping for mysharepoint.domain.com.