C4D can’t open certain .stl files.
After some testing, I think the problem is coming from empty spaces being written into the files by certain applications. The built-in .stl importer seems to not like that.
So I’ve written my own C4D plugin using some code I found on the internet that will import these types of .stl files (binary files only).
It’s not perfect. But it’s good enough in a pinch when dealing with these files that won’t load using the built-in STL importer.
And you can always make any code changes you wish. There’s no usage restrictions on it.