StackOverflow: IIS Express is automatically disabling anonymous authentication for my project, why?
To disable anonymous authentication in IIS Express, I found updating the following element in the project files (i.e. csproj, vbproj) allowed me to debug my application with anonymous access.
Changing the IISExpressAnonymousAuthentication setting to enabled fixes the issue (it can also be done within Visual Studio, select project, F4, set Anonymous Authentication in the properties grid to Enabled).
<IISExpressAnonymousAuthentication>disabled</IISExpressAnonymousAuthentication>