Can you provide details, i.e. a sample file and a piece of self-contained code which reproduces the problem?

Test Excel(xlsx):
   New one Excel file on windows platform, open it, select Cell D4 and set cell value to "=IFERROR(VLOOKUP(C4,'Country List'!$A$1:$B$251,2,0),"")"(except outside ").
 'Country List' is a sheet, contains the conntry name list, col A full contry name, col B abbreviation.


Test Code: 
   @Test
    public void testRenameSheet() throws IOException, InvalidFormatException {
        String filePath = "D:\\win7temp\\testiferror.xlsx";
        Workbook book = WorkbookFactory.create(new File(filePath));
        Sheet sheet = book.getSheetAt(0);
        System.out.printf("origin name:%s \n", sheet.getSheetName());
        book.setSheetName(0, "NewSheetName");
        System.out.printf("rename over!");
    }

Output is:
origin name:Sheet1 

java.lang.NullPointerException
	at org.apache.poi.xssf.usermodel.helpers.XSSFFormulaUtils.updatePtg(XSSFFormulaUtils.java:128)
	at org.apache.poi.xssf.usermodel.helpers.XSSFFormulaUtils.updateFormula(XSSFFormulaUtils.java:98)
	at org.apache.poi.xssf.usermodel.helpers.XSSFFormulaUtils.updateSheetName(XSSFFormulaUtils.java:78)
	at org.apache.poi.xssf.usermodel.XSSFWorkbook.setSheetName(XSSFWorkbook.java:1361)
	at com.lrm.test.POICopyUtilTest.testRenameSheet(POICopyUtilTest.java:36)

Created attachment 32851 [details]
test iferror function on POI 3.11 when set new sheet name.

(In reply to Dominik Stadler from comment #1)
> Can you provide details, i.e. a sample file and a piece of self-contained
> code which reproduces the problem?

I test it on POI 3.12 is ok.