Fixed it.
Turned on Recompute Normals and Use Full Precision UVs under Build Settings in the Mesh Options and then Used the “Generate Unique UVs” to Regenerate my UV Layouts from Channel 0 to Channel 0.
After that the Corners are gone. Even on Lightmap Resolution 64.
And after the Regeneration of the UVs you can simply turn of Recompute Normals and Use Full Precision UVs.
