Extender Provider failed to return an Extender

I encountered the

The Extender Provider failed to return an Extender for this object.

message in a winforms designer window.

I'm aware that your question is about ASP.Net 3.5, but in my case I resolved it by:

• Closing the solution in Visual Studio
• Deleting the obj and bin directories from the problematic project.

When I reopened the solution the error no longer occurred in the designer.

I solved it by just closing Visual Studio and opening it again. I was using Visual Studio 2015 Community Edition and didn't have to delete those folders.

In my case run Visual Studio as administrator helped. My project uses local IIS and Studio needs Admin permissions to manage IIS.