Haven't done the SSCE yet. But one of my colleagues got to talk to the customer again. Apparently it has something to do with the path of jar.
When he moves the application jar to some folder higher in the directory structure (e.g. on his Desktop, which is /Users/christpoh/Desktop/) the application starts and loads the resources without complaining.
I think I'll have to delve into the topic of classloading once more