Okay, I managed to find the solution with the help of Lucas.
First off: That bug was created because UnrealEngine doesn’t replace illegal characters from files inside folder when you import them.
The way to fix your project again is to get a folder with the exact same name into the same location and delete it. UE will then not try to find it anymore.
For me that ment to put a folder called “Standart Boden Textur” into “/Game/Own/Textures/” and delete it again.
I hope that helped. And thanks to Lucas once again.