I just noticed Rhino fails to import Materials with letters like ä, ü, ö etc. - however in the German translation a lot of the default materials have these letters in the file name. Manually changing their name in “ue”, “ae”, “oe” saves the issue.
I have now “solved” the issue by using the Renamer app to change all “ü” to “ue”, “ö” to “oe” and “ä” to “ae”. Would still be nice to have this resolved on the software side as I don’t know if there are any other resources which have been translated this way.