When attempting to Export the XML of attached xlsx, there occurs following exception:
java.lang.IndexOutOfBoundsException: Index: 1, Size: 1
	at java.util.ArrayList.rangeCheck(Unknown Source)
	at java.util.ArrayList.get(Unknown Source)
	at org.apache.poi.xssf.extractor.XSSFExportToXml.exportToXML(XSSFExportToXml.java:203)
	at org.apache.poi.xssf.extractor.XSSFExportToXml.exportToXML(XSSFExportToXml.java:109)

The reason is, that one column of the table isn't mapped. When I do the export from Excel, it works. There is a different handling between Excel and Apache POI.
The code for exporting the XML is as following:

 XSSFWorkbook wb = /*given from previous code execution*/;
       for (XSSFMap map : wb.getCustomXMLMappings()) {
           XSSFExportToXml exporter = new XSSFExportToXml(map);

           ByteArrayOutputStream os = new ByteArrayOutputStream();
           exporter.exportToXML(os, true);
       }