Created attachment 37832 [details]
OOXML workbook for reproducing the bug

I'm getting XmlValueDisconnectedException (see bellow) when setting certain formula cells to blank. Possibly other modifications which are changing formula cells to other types does the same. It happens only with some xlsx workbook.

I've found out that it happens when there are "nested shared formulas" in the sheet definition. Please see the attached workbook where I've created such sheet (using Excel 2010 and copy pasting formula cells).

The test case using the attached workbook is:

 @Test
    void testSetBlankOnNestedSharedFormulas() throws IOException {
        try (XSSFWorkbook wb1 = XSSFTestDataSamples.openSampleWorkbook("testSharedFormulasSetBlank.xlsx")) {
            XSSFSheet s1 = wb1.getSheetAt(0);
            assertNotNull(s1);
            Iterator<Row> rowIterator = s1.rowIterator();
            int count = 0;
            StringBuilder sb = new StringBuilder();
            while (rowIterator.hasNext()) {
                Row row = rowIterator.next();
                Iterator<Cell> cellIterator = row.cellIterator();
                while (cellIterator.hasNext()) {
                    Cell cell = cellIterator.next();

                    // the toString is needed to exhibit the broken state
                    sb.append(cell.toString()).append(",");
                    count++;

                    // breaks the sheet state
                    cell.setBlank();
                }
            }
            assertEquals(10, count);
            assertEquals("2-1,2-1,1+2,2-1,2-1,3+3,3+3,3+3,2-1,2-1,", sb.toString());
        }
    }

The test throws following exception:

org.apache.xmlbeans.impl.values.XmlValueDisconnectedException
	at org.apache.xmlbeans.impl.values.XmlObjectBase.check_dated(XmlObjectBase.java:1243)
	at org.apache.xmlbeans.impl.values.XmlObjectBase.getStringValue(XmlObjectBase.java:1509)
	at org.apache.poi.xssf.usermodel.XSSFCell.convertSharedFormula(XSSFCell.java:494)
	at org.apache.poi.xssf.usermodel.XSSFCell.getCellFormula(XSSFCell.java:472)
	at org.apache.poi.xssf.usermodel.XSSFCell.getCellFormula(XSSFCell.java:445)
	at org.apache.poi.xssf.usermodel.XSSFCell.toString(XSSFCell.java:988)
	at org.apache.poi.xssf.usermodel.TestXSSFSheet.testSetBlankOnNestedSharedFormulas(TestXSSFSheet.java:2040)

I also have a fix for the issue. The problem is in the method XSSFSheet#onDeleteFormula where I've added following line:
if (nextF.getT() == STCellFormulaType.SHARED && nextF.getSi() == f.getSi()) {

I'll try to attach a patch for that...