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XMLSchema contrained files validation is broken.
FIXED in SharedXMLSupport.java new revision: 1.7
More precisely it ignored document's xsi:schemaLocation attribute. Validation set JAXP's schemaLocation property overwriting document's xsi:schemaLocation by a value from URIResolver (Project entity resolver returns nonsences for project namespace URis). Nevertheless document's xsi:schemaLocation attribute must be honored. Now it's 1:1 mapped to the JAXP property.
It's still unreliable behind effective firewall due to issue #53364. Now it can be workarounded by specifing xsi:schemaLocation attribute pointing to local xsd...
Validate of project.xml in WebApp works fine.