وإذا كان في السماء علة قبل الإمام شهادة الواحد العدل في رؤية الهلال
Translation: If there is an obstruction in the sky, the imam accepts the testimony of one upright person regarding sighting the crescent.
Here ʿillah means a visibility impediment—such as cloud or dust—not an illness. When the sky is obstructed, it is unsurprising that most observers fail to see a thin crescent.
That is why Quduri allows testimony from one upright witness in this specified setting. It is not a general rule that one unsupported assertion establishes every religious or legal fact.