validation of jee configuration unreliable

Running "ant validate" in a seam-gen project I am getting the following error or similar error:

validateConfiguration:
[validateConfiguration] http://java.sun.com/xml/ns/javaee/javaee_5.xsd:85:69: schema_reference.4: Failed to read schema document 'javaee_web_services_client_1_2.xsd', because 1) could not find the document; 2) the document could not be read; 3) the root element of the document is not <xsd:schema>.
[validateConfiguration] http://java.sun.com/xml/ns/javaee/javaee_5.xsd:805:49: src-resolve: Cannot resolve the name 'javaee:service-refGroup' to a 'group' component.

Seems that the Xerces parser is unable to download the schema although the schema is available.