Ok, in this case I must have done something wrong elsewhere. But it is a bit confusing, that sometimes it just says "1" (same slide: Constrain(1, φ) = φ), and other times "BDD(1)", so I thought there must be a difference. And then, slide 108 in this chapter, it says "Exists(L, φ) = φ (L must be true here)", so that's just the same, right? L should be 1 here as well.
Thanks anyway for the quick answer, I will try it again, maybe I can figure it out and find my mistake, now.