Only Rambam knew how to learn! (Vos zoll ich tun, nor der Rambam hot gekent lernen)

I heard this story about 40 years ago and it made an indelible impression on me. It was told to me by Rabbi Shlomo Shapiro A”H, the son in law of the pre-war Rav of Cracow, Rav Kornitzer, who heard it from his brother in law, who was present at the event. The Rogatchover Gaon, Rabbi Yosef Rosen (1858-3 March 1936), at the end of his life was suffering from colon cancer (if I remember correctly) and was on his way by train to Vienna for treatment. He eventually died there after an unsuccessful surgery. The Rogatchover was in great pain during the trip and at some point commented that he cannot understand why he deserves this punishment. Did he not spend all his life on learning Torah? What sin could be the reason for this terrible pain? He dozed off for a while and suddenly sat up and said: “I know why I deserve this. All my life I worked on Rambam. What about Rashi, Rabbeinu Tam, Ramban and the other Rishonim? I definitely deserve this.” He dozed off again and a while later sits up again and says: “Ober vos zoll ich tun? Nor der Rambam hot gekent lernen!”. “What should I do? Only Rambam knew how to learn!”