Hi Stephen,
thanks for your answer! I used the verify option in the game launcher for the 4.21.0 installation and after 5 minutes of operation and a restart of the engine the materials loaded correctly. YEAH! The label “Preview 4” was gone today without updating. 
Idk why. You mentioned I should look for the latest Datasmith impoter plugin, but 'm not sure how I can update the plugin separately? I attached a screenshot of my current version.
cheers Rob