"assimp(error): Unable to open file" when loading .obj-files


when I try to load my custom CAD models (.obj) into the Panda3D environment the models are loaded but without textures and materials, they just appear white.

I use following code to load the model (I provide “custom_model.obj” and “custom_model.obj.mtl”):

self.environment = self.loader.loadModel("models/custom_model.obj")

and get the following error back:

Known pipe types:
(all display modules loaded.)
:assimp(error): Unable to open file "/.../models/custom_model.obj".

Does somebody have any ideas how to solve this problem?

Thanks in advance!
Kind regards