I read that Das Ganu was struggling with a scripture. What was the text, and what was his specific problem as explained in Chapter 20?

Yes, Chapter 20 details Das Ganu's challenge. He was working on a commentary in simple Prakrit language called the "Ishavasya-Bhavartha-Bodhini," which was a translation and explanation of the Ishavasya Upanishad for people unfamiliar with Sanskrit. Although he had translated the text word for word, he felt that he had failed to grasp its "internal secret" or deeper meaning. As the Satcharitra states, because he couldn't understand the profound essence of this Upanishad, his "mind found no joy" in the work, which led him to seek Sai Baba's guidance in Shirdi.