Thanks to @3dRaven I could fix it for my project!
I had three Paragon Character in my project: Aurora, Kwang and Gideon. Once I added Gideon Unreal would consistently crash on startup. At that point even when i removed the folder for Gideon in the file explorer it would still crash.
However, I removed the MaterialParameterCollection for Kwang and Gideon (also in the file explorer) and could then open the project again. I kept the one from Aurora.
Ofc now all Materials for Kwang and Gideon were broken. However, I now could go through the materials on Kwang and Gideon and assign the missing MaterialParameterCollection that I still had in my project bc I did NOT delete the one for Aurora (see screenshot).
It was suprisingly little work since they all derive from some base materials. It would probably be best if the “Global” folder is reused for all of the paragon characters but for now my project starts again and my materials work so im moving on.