Can try this procedure:

Next time you run your project with the debugger (normal F5 debugging), it should still remember this setting, and debug with the right debugger.
I think Visual Studio somehow remembers what debugging engine was used last time against a particular executable. If the debugger starts with the wrong debugger it won't work. By default (before the add-in is loaded) Visual Studio detects only the native code, or maybe it detects the wrong type of .NET runtime.