Messed-up lightmapping

Good day,

I’m currently experiencing some issues with a custom mesh i’m trying to import into my game. Since i’m a bloody beginner in 3D-modelling and the stuff that comes with it, lightmapping brings some issues.
I went through the following steps:

Mesh with no UV-Map and therefore no lightmap:

both channels are empty:

As you can see there’s literally no lightning at all after lightning-build.

I went on auto-creating a simple UV-Map in blender and left the lightmapping-job to unreal:

uv-map channel:

lightmap channel created by unreal:

Today i tried creating a lightmap as a second uv-map in blender with the following results:

uv-map (the same as before):

blender-created lightmap:

In comparison:

As you can see there are still plenty of rendering errors although the black parts vary from the blender-lightmap to the unreal-lightmap. Any help would be greatly appreciated! Thanks in advance.