Could not load type System.ServiceModel.Activation.HttpModule assembly System.ServiceModel, Version = 3.0.0.0, Culture = neutral, PublicKeyToken = b77a5c561934e089.

Well, this error took me a lot of time to understand. The message is completely misleading.
In my case the problem was that IIS had two versions of ServiceModel. For some reason the application was trying to load System.ServiceModel.dll version 3.0.0.0 , but was "suprised" by the version 4.0.0.0.
So, what was my solution? I removed the version 3. Did something crash? Not at all. Everything is working fine.
Never had the error again.
Há!