The musical and philosophical ideas behind the opera had begun in Tippett's mind several years earlier. The story, which he wrote himself, charts the fortunes of two contrasting couples in a manner which has brought comparisons with Mozart's The Magic Flute. The strain of composition, combined with his continuing responsibilities at Morley and his BBC work, affected Tippett's health and slowed progress. Following the death in 1949 of Morley's principal, Eva Hubback, Tippett's personal commitment to the college waned. His now-regular BBC fees had made him less dependent on his Morley salary, and he resigned his college post in 1951. His farewell took the form of three concerts which he conducted at the new Royal Festival Hall, in which the programmes included A Child of Our Time, the British première of Carl Orff's Carmina Burana, and Thomas Tallis's rarely performed 40-part motet Spem in alium.In 1951 Tippett moved from Limpsfield to a large, dilapidated house, Tidebrook Manor in Wadhurst, Sussex. As The Midsummer Marriage neared completion he wrote a song cycle for tenor and piano, The Heart's Assurance. This work, a long-delayed tribute to Francesca Allinson (who had committed suicide in 1945), was performed by Britten and Pears at the Wigmore Hall on 7 May 1951. The Midsummer Marriage was finished in 1952, after which Tippett arranged some of the music as a concert suite, the Ritual Dances, performed in Basel, Switzerland, in April 1953. The opera itself was staged at Covent Garden on 27 January 1955. The lavish production, with costumes and stage designs by Barbara Hepworth and choreography by John Cranko, perplexed the opera-going public and divided critical opinion. According to Bowen, most "were simply unprepared for a work that departed so far from the methods of Puccini and Verdi". Tippett's libretto was variously described as "one of the worst in the 350-year history of opera" and "a complex network of verbal symbolism", and the music as "intoxicating beauty" with "passages of superbly conceived orchestral writing". A year after the première, the critic A.E.F. Dickinson concluded that "in spite of notable gaps in continuity and distracting infelicities of language, [there is] strong evidence that the composer has found the right music for his ends".Much of the music Tippett composed following the opera's completion reflected its lyrical style. Among these works was the Fantasia Concertante on a Theme of Corelli (1953) for string orchestra, written to commemorate the 300th anniversary of the composer Arcangelo Corelli's  birth. The Fantasia would eventually become one of Tippett's most popular works, though The Times's critic lamented the "excessive complexity of the contrapuntal writing ... there was so much going on that the perplexed ear knew not where to turn or fasten itself". Such comments helped to foster a view that Tippett was a "difficult" composer, or even that his music was amateurish and poorly prepared. These perceptions were strengthened by controversies around several of his works in the late 1950s. The Piano Concerto (1955) was declared unplayable by its scheduled soloist, Julius Katchen, who had to be replaced before the première by Louis Kentner. The Dennis Brain Wind Ensemble, for whom Tippett had written the Sonata for Four Horns (1955), complained that the work was in too high a key and required it to be transposed down. When the Second Symphony was premièred by the BBC Symphony Orchestra under Boult, in a live broadcast from the Royal Festival Hall on 5 February 1958, the work broke down after a few minutes and had to be restarted by the apologetic conductor: "Entirely my mistake, ladies and gentlemen". The BBC's Controller of Music defended the orchestra in The Times, writing that it "is equal to all reasonable demands", a wording that implied the fault was the composer's.

Question: What was the name of Tippett's work that was deemed unplayable by it's scheduled soloist?
Answer: The Piano Concerto