I have been fighting issues with this exception as well for last two weeks. I was excited to see the 2.1 release hoping this fix it but it doesn't appear to be the case.
The first thing I noticed (which seems to not play well with unity or mef) is a huge descrepancy in the versions of the Smooth Streaming DLL. Different projects and sources referenced around this community are usually one of two flavors:
Version 3.0.711.19 - which happens to be OLDER than the Version 1.0.837.34 - which is what Microsoft 'officially' released. Version 3.0.711.19.
This may or may not be the culprit. Then again it could be the version of MEF or Unity your app is using. The souce code branches aren't updated or aligned with the drops and releases and further increases the complexity of ironing out the root cause.