The key connecting point to understanding what critical theory generally identifies as Mr. Nilson's ailment in "The Japanese Quince" is that, in the story, the ailment comes upon him suddenly on the first spring morning as he opens a window: "spring at last!" Tellingly, it occurs again shortly later when he performs the antithetical action of picking up the morning newspaper, undoubtedly to read the financial section to corroborate his musings on the "price of Tinto."
Critics generally agree that Galsworthy's central theme is the personal physical and mental deterioration that inevitably occurs when a person disassociates from exposure to and appreciation of nature. Under the aegis of this theme, Nilson's ailment would be the signs of inward deterioration following years of isolation from nature, an isolation (1) that is reflected in Nilson's alterego doppleganger neighbor, Mr. Tandram (similar to "tandem") and (2) that renders him looking a little foolish in his awkward and unaccustomed contemplation of a spring blooming quince tree, which he cannot admire until the correct species is known.
Galsworthy believed that the human ability to heal was integrally connected to an ability to appreciate nature, which makes the inverse equally true: an inability to appreciate nature leads to an inability to heal. Galsworthy illustrates this by Nilson's ailment, a "queer" "choky" feeling that was a "faint aching just above the heart." He supports his thesis in Nilson's name: nil is "nothing; naught; zero." Mr. Nilson is suffering from a zero connection to nature and as a result is becoming a nothing, naught, with a ache where his heart should be and a choke where his breath of life should be. All of which returns our consideration to the neat opening gambit of the story in which Nilson is beset with his ailment on the first day of spring when nature is renewed.