The answer turned out to be simple: The instructions in the link http://go.microsoft.com/fwlink/?LinkId=179545 given in the error message really worked. One has to tell VS that these files are 'trusted', so
- close VS
- right-click on the .dll's and select 'properties'
- in the 'general' Tab, press the 'unblock' button
What is really weird and kind of hilarious to me is that VS will let me work with these 'untrusted .dlls' just fine in c# code, but not in .xaml ...