I found the source of the problem though, it was the fact that this whole thing consists of a plurality of different projects. The communication module of it contains multiple different libraries to separate service providers. The returned "PersonData" resided in one of these assemblies and was not referenced from the main app project. The error I received didn't really reflect that this was the root cause.
So basically the error I received gave very little hint towards what was actually wrong with the solution. Hope this helps someone else out there receiving the same error. :P