URP - Directional Light Flickering

I just updated to Unity 2019.3.9f1 and added URP (after a lot of headaches), the effect seen in the photo below was caused after Upgrading my materials to Universal RP Materials (this cannot be reverted, however I did backup my project before updating the Materials). I am also receiveing a constant error spam of

Converting invalid MinMaxAABB
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

each time it flickers.

same here. It worked for months with URP. Now I started preparing for shipping and play arround with addressables and since yesterday some parts of the static objects which are created by Instantiate(aPrefab) do not get lit when camera moves arround.
What I found out is, that when I change the shader of an instatiated object from simpleLit ti lit, it works all of a sudden. But it does not help to do this in the material before hitting play.
I am currently sitting and waiting for all assets to re-import in the hope that will help.

(sidenote: Due to the stupid progress bar this may take about 30 minutes or 10 hours. Best is to just go away. )