Update: I saw where turning off the setting "DeepScreen" would likely help with this because it's purpose is to interogate those apps (.exe and I'm guessing your IIS .Net .dll's). I was pretty sure I had turned that off the 1st time I tried version 9 (2014), but I removed version 7 and tried 9 again, with that turned off.
That setting is in the main settings screen under the "Avast" category. I also turned off "Reputation services" right above that setting.
This solved it for me with Visual Studio and standard WinForms apps. I usually do IIS asp.net apps but I'm on a detour right now with this WinForms app. I say that because if it works for me running the WinForms .exe app in debug mode, it'll probably work for you with IIS.
Give it a try. I've rebooted several times and Avast has updated definitions several times, and it's still working fine with Visual STudio now.