Die Krähen behaupten, eine einzige Krähe könnte den Himmel zerstören. Das ist zweifellos, beweist aber nichts gegen den Himmel, denn Himmel bedeuten eben: Unmöglichkeit von Krähen.
The crows maintain that a single crow could destroy the heavens. There is no doubt of that, but it proves nothing against the heavens, for heaven simply means: the impossibility of crows. [Kaiser/Wilkins]
The crows like to insist that a single crow is enough to destroy heaven. This is incontestably true, but it says nothing about heaven, because heaven is just another way of saying: the impossibility of crows. [Hofmann]
The purpose of this aphorism might be to set dialectical reasoning running round in circles until it falls exhausted.
I don't know how to approach this aphorism without taking the stupid interpretation as my starting point.
Stupid: the crows are the doubters who deny heaven, but, since heaven is faith-in-heaven, they end up denying it to themselves. So, believe etc.
What does Kafka do to prevent a stupid interpretation? For one thing, the crows aren't denying heaven, they are asserting they can destroy it. For another thing, why crows? The hallmark of bad readings of Kafka is the assumption that he writes allegories. The crows are crows, and he must have chosen crows because they live in the sky. The word Himmel means both heaven and sky.
From the point of view of the crows, they coexist with heaven. From heaven's point of view, there's no such things as crows. This is another stab at a model of good and evil, or the positive and the negative, of the kind Kafka has been working on in other aphorisms. He's trying out different terms for these two sides, and experimenting with alternative renderings of their relationship. Evil insists on its parity with good, but good does not insist on its parity with evil, in fact, good is the absence of evil, but only in thought.