According to Chapter 20, what was the specific problem Das Ganu faced while working on the Ishavasya Upanishad?

The specific problem Das Ganu faced is detailed in Chapter 20. He had started a commentary called "Ishavasya-Bhavartha-Bodhini" to explain the meaning of the Ishavasya Upanishad in simple Marathi for those who didn't know Sanskrit. Although he completed a word-for-word translation, the chapter states that "without understanding the internal secret, the mind found no joy." A particular doubt arose that he couldn't resolve through his own scholarship. The text emphasizes that the Upanishads are full of deep meanings, and his inability to grasp this hidden essence is what prompted him to take his doubt to the feet of his Satguru, Sai Baba, in Shirdi.