Export space problem

When I try the XML export (of a space) I get this stacktrace:

The following error(s) occurred:

• com.atlassian.confluence.importexport.ImportExportException: Couldn't backup database data. at com.atlassian.confluence.importexport.impl.AbstractXmlExporter.backupEntities(AbstractXmlExporter.java:199) at com.atlassian.confluence.importexport.impl.AbstractXmlExporter.backupEverything(AbstractXmlExporter.java:78) at com.atlassian.confluence.importexport.impl.AbstractXmlExporter.doExport(AbstractXmlExporter.java:55) at com.atlassian.confluence.importexport.impl.XmlExporter.doExport(XmlExporter.java:24) at com.atlassian.confluence.importexport.DefaultImportExportManager.exportAs(DefaultImportExportManager.java:84) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:324) at
  .....

java.lang.Thread.run(Thread.java:534) Caused by: java.lang.ClassCastException at com.atlassian.confluence.importexport.impl.XMLDatabinder.renderOtherType(XMLDatabinder.java:271) at com.atlassian.confluence.importexport.impl.XMLDatabinder.renderProperty(XMLDatabinder.java:242) at com.atlassian.confluence.importexport.impl.XMLDatabinder.toGenericXML(XMLDatabinder.java:153) at com.atlassian.confluence.importexport.impl.AbstractXmlExporter.backupEntities(AbstractXmlExporter.java:192) ... 109 more