Luckily I found it, I checked the materials from the AcientContent, because I thought, hm this stuff might be outdated with the new UE5 update and it was indeed the Debug material from the AcientOneCore.
I still get a crash when running my game, but this is caused, by another issue, it happens while loading a sequence. But this is a topic for a new thread, for me, this one is solved! Thanks for the support anyway!