BIRT exception

vlpe

I have an application hosted in tomcat using BIRT Engine. When initializing BIRT using the following code
DesignEngine designEngine = new DesignEngine(designConfig);
modelSession = designEngine.newSessionHandle(ULocale.ENGLISH);
an error occurs :
SEVERE: java.lang.ClassCastException: org.eclipse.birt.report.item.crosstab.core.de.CrosstabItemFactory cannot be cast to org.eclipse.birt.report.model.api.extension.IReportItemFactory. Does somebody know why this error occurs and how to solve it? Thanks in advance.