Posted by Battling for hours an empty Reference.cs class. Found a good checklist (which summarizes what is usually the problem when I run into this problem)
http://chrisdomino.com/blog/post/WCF-Service-References-Generating-Empty-Proxy-Classes
Went through all my usual checklists – but then by carefully selecting which data classes to reuse in the service reference configuration – the “Update service reference” worked again. Not sure if the update process was implicitly dragging some incompatible classes into the picture …? But it is fixed now 
