[SOLVED] I can no longer debug Office Add-in from Visual Studio

I can no longer debug Office Add-in from Visual Studio

I have been working on an add-in for Excel. When I run my add-in from Visual Studio, it usually launches Excel, and adds a button the the ribbon which I use to open a task pane.

Now, when Excel launches, it immediately shows a task pane with the following error.

ADD-IN ERROR

This add-in is no longer available: add-ins inserted during development are only available during debugging from Visual Studio. Please open your project in Visual Studio and re-run your application, or deploy your add-in into a valid catalog and re-insert.

The suggestion in the error message is frustrating because it is telling me to do exactly what I am trying to do.

I have:

Restarted my Computer
Cleared the Office Cache
Modified the version number in the manifest
Validated the manifest
Removed Task Pane Add-ins using the Document Inspector

My manifest has not changed at all since the plugin last worked. (save for my debugging attempts)

I am using Visual Studio 16.6.0 Preview 6.0, and Excel for Office 365 16.0.12527.20260

Solution

I found the source of the issue.

I had a Start Document specified in my Add-in project. When I switched the Start Document back to [New Excel Workbook], my add-in started loading correctly.

I tried opening my document in Excel, and I saw that it was marked as Read-Only. It appears that Visual Studio cannot launch an Excel add-in with a read-only start document. I resaved my file, and set it back to being the Start Document of the add-in project. Now, everything has started working again.