I got the message : Exception in thread "main" org.apache.poi.openxml4j.exceptions.OpenXML4JRuntimeException: Rule M2.4 exception : this error should NEVER happen! Please raise a bug at https://bz.apache.org/bugzilla/enter_bug.cgi?product=POI and attach a file that triggers it, thanks!
	at org.apache.poi.openxml4j.opc.internal.ContentTypeManager.getContentType(ContentTypeManager.java:343)
	at org.apache.poi.openxml4j.opc.internal.ContentTypeManager.removeContentType(ContentTypeManager.java:256)
	at org.apache.poi.openxml4j.opc.OPCPackage.removePart(OPCPackage.java:943)
	at org.apache.poi.openxml4j.opc.PackagePart.getOutputStream(PackagePart.java:522)
	at org.apache.poi.xssf.usermodel.XSSFWorkbook.commit(XSSFWorkbook.java:1678)
	at org.apache.poi.POIXMLDocumentPart.onSave(POIXMLDocumentPart.java:341)
	at org.apache.poi.POIXMLDocument.write(POIXMLDocument.java:206)

Since the file that triggers it is fairly big I'm attaching the code that triggers it.
This code is the very beginning, the only lines before it are declarations and initializations. Written in Java 8 u 74. Good luck ! :)

for(String folder : godine){
            XSSFWorkbook wbTemp = new XSSFWorkbook();
            wbTemp.createSheet();
            FileOutputStream fo = new FileOutputStream(lokacijaResursa+"\\statistika-"+folder+".xlsx");
            HashMap<String,Integer> pozicija = new HashMap<>();
            napraviCelije(wbTemp,100,200);
            for(int i=1;i<=12;i++) {
                try {
                    XSSFWorkbook mjesec = new XSSFWorkbook(new FileInputStream(new File(lokacijaResursa + "\\" + folder + "\\" + i + ".xlsx")));
                    XSSFSheet sheetMjesec = mjesec.getSheetAt(0);
                    popuni(wbTemp, mjesec, pozicija, i);
                }
                catch (Exception ex){}
            }
            wbTemp.write(fo);
            fo.close();
            XSSFWorkbook wb = new XSSFWorkbook(new FileInputStream(lokacijaResursa+"\\statistika-"+folder+".xlsx"));
            int numOfCells = 0;
            try{
                for(int i=3;;i++){
                    Row r = wb.getSheetAt(0).getRow(1);
                    Cell c = r.getCell(i,Row.CREATE_NULL_AS_BLANK);
                    if(c.getCellType() == Cell.CELL_TYPE_BLANK)
                        break;
                    numOfCells++;
                }
                for(int i=2;;i++){
                    Row r = wb.getSheetAt(0).getRow(i);
                    if(r.getCell(0,Row.CREATE_NULL_AS_BLANK).getCellType() == Cell.CELL_TYPE_BLANK)
                        break;
                    for(int j=3;j<numOfCells;j++){
                        Cell c = r.getCell(j,Row.CREATE_NULL_AS_BLANK);
                        if(c.getCellType() == Cell.CELL_TYPE_BLANK)
                            break;
                        else c.setCellValue(0);
                    }
                }
            }
            catch (NullPointerException ex){
                System.out.println("Nema fajlova");
                System.exit(0);
            }
            wb.close();
            fo = new FileOutputStream(new File(lokacijaResursa+"\\statistika-"+folder+".xlsx"));
            wb.write(fo);
            fo.close();
        }