Strange shadows appear on object near camera

Hi,

I'm developing a 3D space shooter and I want dynamic shadows I already have my space ship and simple asteroid objects, but when I move near them weird shadows appear on them:

This only occurs when realtime shadows are activated. I tinkered with a lot of the lighting and shadow settings but to no avail. All 3D model assets are created with blender. My project actually is open source so you can actually just clone the git repo and see for yourself
https://gitlab.com/robertbleyl/drone-control

I suspect that there is something wrong with the asteroid models since I'm not particularly fluent with blender and the importing of blender files into unity - so here a direct link to the models folder: https://gitlab.com/robertbleyl/drone-control/-/tree/develop/Assets/Models

I'm also confused about the whole light maps thing in the unity editor TBH: when I select the blender models in the editor and check "generator lightmap uvs" and then try to generate them in Window -> Rendering -> Lighting settings no lightmaps appear to be generated (it says "0 B" and "No Lightmaps"). What am I doing wrong?

One more important info: I'm using Unity 2019.3.5f1 Personal for Linux (ubuntu 18.04).

If I should provide any more details feel free to ask. Any help would be much appreciated!

 

So I did some experiments with a very simple blender model (a cube with 2 extruded faces and a simple grey material) and the same issue occurs. When I use a simple Unity cube for my asteroids the issue does not occur. So this tells me that something with the blender import is not right, but I don't know what. I recalculated the outside normals again but it didn't change anything.
Any ideas?