Hi Th3Gi4nt. I think problem in face normal, check it in Cinema 4D’s, and set to outside orientation. Here you can find this problem with images. Importing a model - Rendering - Epic Developer Community Forums
Hello guys, I’ve been having a very interesting / frustrating problem lately. I have a skeletal mesh which I edited in Cinema 4D and the result is shown in the first picture. I was feeling kinda bored to rig him myself in Cinema 4D, so uploaded him and rigged him in Mixamo (Picture 2). Everything is ok as of yet.
When I import him into the Unreal Engine 4 editor I encounter no problems. BUT when I open the skeletal mesh to fix the materials and textures to fit (he is completely white when I import him), I see this (Picture 3,4). Initially it seems his whole body is reversed but with a closer look I can see that that’s not the case. Instead from the front what I can see is the interior of the back and from the back what I can see is the interior of the front. That is the case for each and every body part.
I also imported him without rigging him to see if he as a static mesh would still seem like that (Picture 3,4) and he DID. So it’s not Mixamo’s fault. It’s not Cinema 4D’s fault as well, since I did the same process on Maya. So I guess it’s something with the engine. I promise I haven’t touched anything in the engine I wasn’t supposed to 
Do you guys know what has happened to him? How can I fix this?
Good point Anatoliy. I flipped the normals and that made him visible. But now he’s just a bunch of triangles, looking terribly. Got any tip for that? Because in his current state he’s useless… Open pic for a better look.
You need smoothing normals.
For better look if in Cinema4D have angle of smooth normals. Try from 30 to 45 angle. And look what will be better.
Yep, a 30 degree smoothing angle did the trick. Thanks a lot for the heads up Anatoliy!