Lumen wierd artifacts

Hello! I don’t use Unreal on a regular basis. I just started exploring Unreal 5. Working in 3D, I’d like to use Lumen to render my animations.
My first contact is a bit contrasted… I tried to build a scene and render some test animations but I get lots of problems… Light flickering, It probably comes from using emitting material as lights. I will replace the ones in the far back (against the wall) with Unreal Rectangular lights.
Now, the real problem comes from those black squarish artefacts showing at the beginning as you can see in the short video.
https://youtu.be/tfTZJNAKy9k

Any suggestions?
Thank you!

I rendered several outputs/passes to try and narrow down the culprit. The black squares appear ONLY in the detailed lighting pass, not the general lighting output. So I guess It has to do with those specific settings…

Multiple Changes to the settings make no difference. I’m lost… did anybody come across a similar issue? Any suggestion?