Ok - This is Murphys Law I guess... I have been debugging this for two days without result and 30 min after posting this question I found the bug.

In claRenderobj I first bind the _VAO . The second line should have been to the vertexbuffer _buffer.

Simple misstake that was quite hard to debug.

        GL.BindVertexArray(_VAO);
        GL.BindBuffer(BufferTarget.ArrayBuffer, _VAO);