Visual Studio debugger doesnt use the same virtual path you app will end up using.
Try running your app from IIS and you will see everything works smooth.
Then, there are a couple solutions for what you're getting.
1. Dont use "/" to say root. Try to use relative paths whenever you can
2. From now on, you refer to the root as ~/ (in aspx pages)
I also think there's a way to adjust this behaviour in VS but the behaviour you're getting is actually correct. It is also good to prepare your apps to run "from everywhere" in a directory structure.
Regards,
Mauro