I have a project that’s been running for 15 years that is suddenly giving an error when trying to create an Excel file on both my local machine and my user’s machine at their office. To test, I’ve created a new project (Windows 10, Windows Forms, .Net Framework 4.7.2), installed the Microsoft.Office.Interop.Excel NuGet package, added a button to a form and in the click event, set the value of a cell to “Hello World” and I’m still getting the error. I can’t find anything online about this error either outside of suggestions to repair Excel which I’ve already done to no avail.