I hope i am in the right place here, but i think my problem has to do with Lumen.
As you can see in the Picture, Light bleeds trough the Edges of the Roof and some weird shadowy lines/noise is going on.
I tried about everything i could find in Forums regarding light bleeding, but its behaviour in my case is so strange, i couldnt find a similiar case.
The thing is, that visual “bug” seems to only occur on one side of a symmetrical mesh. Furthermore the issue disappears, if i move very close with the camera (it gradually gets better, altough a weird circular shadow becomes visible, and at some point close to the wall, the lighting is fine).
As for the setup of the scene:
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The Mesh is one piece (i know it should be split up meshes for Lumen, but that makes the issue much worse and affects every edge). It has a thickness of 40 cm.
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The lighting consists mainly of a directional light at the reccomended 120.000 Lux for direct Sunlight and a Skylight (and Height Fog, VolumetricCloud and SkySphere).
The Issue appears the most, when the Directional Light is coming from directly above.
I tried for hours on end, everything i could think of or read in other light bleed issue threads, played around with every slider i could find in all light sources, Mesh, Project Settings and PostProcessVolume.
If anyone has an idea or even solution at hand, thanks in advance. 
Cheers
Edit:
I read somewhere, turning on Raytracing might help, but i only have a GTX 1080. So that is not an option.
Also in the time this post was pending, i tried the whole “splitting up the mesh into simple parts” again.
For some reason, as i expected from previous tests, it made everything worse.
Almost the whole “House” is now black in the Lumen View. Everything is lit with SSR, wich looks… bad.
In some spots, the same phenomenon as in the other House happens, where a bit light bleeds trough and the weird shadowy lines/noise appear. In the Lumen Scene, these Edges are partially lit.
Edit hours later:
I’ve taken a look at the Starter Architecture Meshes in Unreal itself, because they appear to work just fine with Lumen.
So i’ve made the effort (honestly it was fun), to build the House again, with Wall Elements in the Style of the Starter Assets from Unreal.
I also Unwrapped the UVs (i think properly) in Blender as a Lightmap.
Lumen works a lot better now, but the initial Problem described above still persists and Lumen still has Dark and even Pitch Black Corner/Parts of a Wall on the Building.
On the last Picture, the edge over the window, shows the same visual issue as the very first image of this post. Also one of the dark corners is visible in the upper left, where it seems to use SSR.
I hope someone can help. Every Tutorial for Lumen always has perfect lighting, without even a mention what to do with the mesh to achieve this Quality of Lighting.