Excel and “unreadable content” when creating an Open XML spreadsheet with MemoryStream

That error message means that the XML that makes up your excel document is not conforming to the XML Schema and is invalid. You can use the Open XML SDK 2.0 Productivity Tool to see where the issue is located.

I also copied the code from the bottom of your link and got it to work like Chris said in his comment. Does your code look like this below?

// Open the document for editing.
using (SpreadsheetDocument spreadSheet = SpreadsheetDocument.Open(docName, true)) 
{
    // Add a blank WorksheetPart.
    WorksheetPart newWorksheetPart = 
       spreadSheet.WorkbookPart.AddNewPart<WorksheetPart>();
    newWorksheetPart.Worksheet = new Worksheet(new SheetData());

    Sheets sheets = spreadSheet.WorkbookPart.Workbook.GetFirstChild<Sheets>();
    string relationshipId = 
       spreadSheet.WorkbookPart.GetIdOfPart(newWorksheetPart);
    // Get a unique ID for the new worksheet.
    uint sheetId = 1;
    if (sheets.Elements<Sheet>().Count() > 0)
    {
        sheetId = 
        sheets.Elements<Sheet>().Select(s => s.SheetId.Value).Max() + 1;
    }

    // Give the new worksheet a name.
    string sheetName = "Sheet" + sheetId;

    // Append the new worksheet and associate it with the workbook.
    Sheet sheet = new Sheet() 
    { Id = relationshipId, SheetId = sheetId, Name = sheetName };
    sheets.Append(sheet);

    string docName = @"C:\Users\Public\Documents\Sheet7.xlsx";
    InsertWorksheet(docName);
}

// Given a document name, inserts a new worksheet.
public static void InsertWorksheet(string docName)
{
    // Open the document for editing.
    using (SpreadsheetDocument spreadSheet = SpreadsheetDocument.Open(docName, true))
    {
        // Add a blank WorksheetPart.
        WorksheetPart newWorksheetPart = spreadSheet.WorkbookPart.AddNewPart<WorksheetPart>();
        newWorksheetPart.Worksheet = new Worksheet(new SheetData());

        Sheets sheets = spreadSheet.WorkbookPart.Workbook.GetFirstChild<Sheets>();
        string relationshipId = spreadSheet.WorkbookPart.GetIdOfPart(newWorksheetPart);

        // Get a unique ID for the new worksheet.
        uint sheetId = 1;
        if (sheets.Elements<Sheet>().Count() > 0)
        {
            sheetId = sheets.Elements<Sheet>().Select(s => s.SheetId.Value).Max() + 1;
        }

        // Give the new worksheet a name.
        string sheetName = "Sheet" + sheetId;

        // Append the new worksheet and associate it with the workbook.
        Sheet sheet = new Sheet() { Id = relationshipId, SheetId = sheetId, Name = sheetName };
        sheets.Append(sheet);
    }
}

Based on the XML of the worksheet, this part:

<x:row r="1" spans="1:37">
    <x:c r="A1" t="s">
        <x:v />
    </x:c>
</x:row>

should not exist outside of the SheetData element. In fact, there appears to be a duplicate, since

<x:row>
    <x:c r="A1" t="inlineStr">
        <x:is>
            <x:t>TestResultFileId</x:t>
        </x:is>
    </x:c>
</x:row>

also exist. Note that the "actual" header Row has no RowIndex assigned, but according to the CellReference of "A1", this particular row is on row 1.

Note that this:

var rows = sheetData.Elements<Row>().Where(r => r.RowIndex > 1).ToArray();

will probably ignore any Row objects without the RowIndex assigned (I didn't test this though...). Which can happen. Excel should have assigned a value, but any third-party software isn't bound to do so (because the Open XML specs state that RowIndex is an optional attribute).

I don't know why there's a Row object outside of SheetData. Check that the original template Excel file doesn't have this "Row object outside of SheetData object" case. If it does, then the original template file was faulty in the first place.

You might want to consider the option of storing the 1st Row in a separate variable first. Then wipe out all the child elements of SheetData. Then Append() that 1st Row in. This might be easier. You can wipe out children like so (insert bad parenting joke here):

sheetData.RemoveAllChildren();