Extender Provider failed to return an Extender

Question

We upgraded from .NET 2.0 to .NET 3.5. A co-worker of mine is getting the following dialog when attempting to run the ASP .NET web project under the debugger in Visual Stud

Answer 1

I had this issue in Visual Studio 2015, the error kept persisting even after cleaning the project, restarting visual studio and restarting my pc.

What resolved it for me, was right-clicking the xaml file and choosing Open with... > XML Editor. It opened the file and the error didn't occur after that.