I gets this bug :
org.apache.xmlbeans.impl.values.XmlValueDisconnectedException
at org.apache.xmlbeans.impl.values.XmlObjectBase.check_orphaned(XmlObjectBase.java:1213)
at org.openxmlformats.schemas.spreadsheetml.x2006.main.impl.CTCellImpl.getR(Unknown Source)
at org.zkoss.poi.xssf.usermodel.XSSFRow.onDocumentWrite(XSSFRow.java:460)
at org.zkoss.poi.xssf.usermodel.XSSFSheet.write(XSSFSheet.java:2743)
at org.zkoss.poi.xssf.usermodel.XSSFSheet.commit(XSSFSheet.java:2712)
at org.zkoss.poi.POIXMLDocumentPart.onSave(POIXMLDocumentPart.java:313)
at org.zkoss.poi.POIXMLDocumentPart.onSave(POIXMLDocumentPart.java:317)
at org.zkoss.poi.POIXMLDocument.write(POIXMLDocument.java:173)
at org.zkoss.zss.app.file.FileHelper.saveSpreadsheet(FileHelper.java:194)

to reproduce follow these steps :
1. insert one column at "F"
2. resize the column "F"
3. change column G font color to red
4. export the excel by click save button
5. stretch G13 to F13 (copy the G13 to F13)
I try it on demo page http://zssdemo.zkoss.org/zssdemo/index.zul
and each time I have this bug