When referencing an XML schema that uses an import of another XML namespace, IDEA doesn't correctly handle the types from the import. The difference between this issue and other XML schema issues is that the imported XML schema uses unqualified elements. I'll attach the Savant example XSDs and the instance document that doesn't work. The strange part is that IDEA understands that what the elements should be, but complains that they are invalid.

Also, here is the W3C doc that covers this type of import where the element types are all unqualified:

http://www.w3.org/TR/xmlschema-0/#import

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