Ok I have no idea what is happening here. The back side of my faces shine through the face in front. I activated "show face normals" so it is better to see. First I thought my typology is mto bad, but it even shows up on a simple cube.I may change something I'm not aware of... I would appreciate your help.
Just in case someone gets the same problem. I've got the reason and solution. The shading issue is caused by a very low clip start value of the viewport while beeing zoomed out. Sub-D and Solidify making it even worse. So nothing crashed just keep the setting high enough if you want to zoom out^^