Thanks for your help!
I tried both: the OBJ loader and the AC3D loader and even and ASE exporter which is originally intended for Unreal custom models. I got the AC3D loader working as far as I can tell now. But I'm still wondering, why the textures (and other stuff) are loaded from some kind of classpath related resource pool. E.g.:
URL url = new String().getClass().getClassLoader().getSystemResource(modelFilename);
After replacing them with a static (new File (modelFilename)).toURL() command, it worked in my environment. But I guess I contaminated the code by that...