"SchemaException: Error loading schema types" when multiple SOAPClient actions invoke the same web service concurrently

Issue

• We have a SOAPProxy, which throws the exception "SchemaException: Error loading schema types" when being initialized with multiple request at the same time. After that error the proxy is unusable.
• All goes ok if the proxy is initialized with only one request.
• Here is the error stacktrace:

2012-12-17 15:54:24,990 ERROR [STDERR] (Thread-330) com.eviware.soapui.impl.wsdl.support.xsd.SchemaException: Error loading schema types
2012-12-17 15:54:24,990 ERROR [STDERR] (Thread-330)     at com.eviware.soapui.impl.wsdl.support.xsd.SchemaUtils.loadSchemaTypes(SchemaUtils.java:176)
2012-12-17 15:54:24,990 ERROR [STDERR] (Thread-330)     at com.eviware.soapui.impl.wsdl.support.wsdl.WsdlContext.loadDefinitions(WsdlContext.java:256)
2012-12-17 15:54:24,990 ERROR [STDERR] (Thread-330)     at com.eviware.soapui.impl.wsdl.support.wsdl.WsdlContext.access$500(WsdlContext.java:53)
2012-12-17 15:54:24,990 ERROR [STDERR] (Thread-330)     at com.eviware.soapui.impl.wsdl.support.wsdl.WsdlContext$Loader.construct(WsdlContext.java:210)
2012-12-17 15:54:24,990 ERROR [STDERR] (Thread-330)     at com.eviware.soapui.support.swing.SwingWorkerDelegator.construct(SwingWorkerDelegator.java:45)
2012-12-17 15:54:24,990 ERROR [STDERR] (Thread-330)     at com.eviware.soapui.support.swing.SwingWorker$2.run(SwingWorker.java:111)
2012-12-17 15:54:24,990 ERROR [STDERR] (Thread-330)     at java.lang.Thread.run(Thread.java:662)
2012-12-17 15:54:24,990 ERROR [STDERR] (Thread-330) Caused by: com.eviware.soapui.impl.wsdl.support.xsd.SchemaException
2012-12-17 15:54:24,990 ERROR [STDERR] (Thread-330)     at com.eviware.soapui.impl.wsdl.support.xsd.SchemaUtils.buildSchemaTypes(SchemaUtils.java:240)
2012-12-17 15:54:24,990 ERROR [STDERR] (Thread-330)     at com.eviware.soapui.impl.wsdl.support.xsd.SchemaUtils.loadSchemaTypes(SchemaUtils.java:172)
2012-12-17 15:54:24,990 ERROR [STDERR] (Thread-330)     ... 6 more