Getting "The designer could not be shown" error even if I create a new project in my solution

[FLDaveM]

In VS2017 Professional (Version 15.9.11), I'm getting The designer could not be shown for this file because none of the classes within it can be designed when I attempt to show a form that has worked for years in a solution loaded with many projects. Yes, the partial class for the form is the first thing in the code file. And no, it's not an inherited form, just your basic form. The same issue appears if I create a new project in my solution and add a form.

If I create a new solution and add a project, I don't get the error. So I don't think the error is with the form, I think it's higher up.

I've tried:

1. cleaning the solution, rebuilding, cleaning, closing VS and reopening it, rebooting
2. updating VS2017 to the latest
3. targeting a different version of .NET (4.6.1)
4. going back to the original version of .NET (4.5)
5. then repeating step 1 with cleaning & rebuilding

The call stack is
at System.ComponentModel.Design.Serialization.CodeDomDesignerLoader.EnsureDocument(IDesignerSerializationManager manager)
at System.ComponentModel.Design.Serialization.CodeDomDesignerLoader.PerformLoad(IDesignerSerializationManager manager)
at Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomDesignerLoader.PerformLoad(IDesignerSerializationManager serializationManager)
at System.ComponentModel.Design.Serialization.BasicDesignerLoader.BeginLoad(IDesignerLoaderHost host)

I've searched and not found any suggestions that help. Again, I think it's in a higher level than the form or even the project.

Now I'm just patiently (or maybe not so patiently) waiting for the Microsoft experts to call me back.

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