This error message indicates that schema validation is still being attempted against the www… url instead of the data… url.
In the patch at Crossref 4.3.6 schema URL changed · Issue #2258 · pkp/pkp-lib · GitHub there is not only a change for the definition in the XML file and PHP files, but also a change to a record in the database:
UPDATE filter_groups SET output_type = 'xml::schema(http://data.crossref.org/schemas/crossref4.3.6.xsd)' WHERE output_type = 'xml::schema(http://www.crossref.org/schema/deposit/crossref4.3.6.xsd)'
Did you manually make all of these changes?