we've had that before i think. It was either :

something to do with the IIS not having a project associated with the app, in which case you need to check the app's directory in IIS setting (Control Panel > Administrative Tools > IIS). Right click on your App and go to properties, the Directory Tab has the application bit at the bottom. Just click Create and this should create an Application. It might be just a virtual directory at present

OR

the project is being started in Releese mode, and not Debug mode

i have a screeny of the 2 different modes, but i can't attach to this message :(. It's a litbox at the top of the screen, Options are :

Debug, Release, Configuration Manager...

hope this helps