I had this problem many years ago when I used C4D R12, but have since lost the post offering a solution.
It has to do with an object in C4D losing textures when I MERGE into a new project.
The textures in question are image maps, not C4D inbuilt textures.
I have a library of objects, downloaded or self created, that I use in my projects, all stored with their image maps in their individual library folder.
If I OPEN the objects into Cinema 4d from the object sub folder in my library, all textures are there as expected.
If I create a new project in a new subdirectory, and MERGE objects into this project from my library, quite often the textures are not imported with the result that I need to go through each texture and re tag the image maps.
If memory serves, it has to do with texture paths, but i cannot find any information on texture paths that would allow me to import without re tagging.