ADD-IN ERROR This add-in could not be started

Question

Using the Visual Studio 2017 C# Excel 2016/2019 Web Add-In template, I'm trying to create a Basic Add-in to "Insert content into Excel spreadsheets [VS option for project]."

All my attempts fail in Excel when I run the project with the message: 'ADD-IN ERROR This add-in could not be started. Close this dialog to ignore the problem or click "Restart" to try again.' Clicking Restart displays a "Loading" dialog in Excel that never completes.

I've unsuccessfully tried all the following steps to resolve this problem:

• Run VS in admin mode
• Switched from Excel 2016 to 2019
• Reset IE settings (ensuring protected mode is on for Internet and Restricted sites)
• Upgraded to Windows Build 1809
• Reset the Excel add-in cache

In addition, I ensured that the localhost SSL certificate is trusted and I can browse to the IIS Express site with no security warnings.

I should note that this issue is happening only with VS2017 templates. If I use node and sideload the add-in, I have no issues on this particular dev machine.

My question is:

• How do I go about getting more information from Excel or VS2017 on why the add-in is not loading and this error?

Answer 1

I finally resolved this frustrating issue. It was caused, most likely, by a damaged Office workload installation in VS2017. I uninstalled and re-installed the workload. I also did a quick office repair before this, but I really think the VS2017 reinstall is what fixed the issue.