EDIT: Srry I found it allready,
The lod terrain was yellow, which means that I have exceeded the max allowable distance of 511 grid between the highest and lowest vertices in one 512x512 patch of LOD.
If you highlight your terrain and hit key V, you will see yellow dots. This means you have made it too high in one spot (maximum distance is 510 units between dots). If you want a steep cliff or moutian, you must use patch mesh.