Complex sentence: His personal problems were often the cause of friction between him and Rodgers; in fact this led to a brief breakup in 1943, at which time Rodgers started working with Oscar Hammerstein II, a school friend of Hart.
Split into simpler sentences: His personal problems were often the cause of friction between him and Rodgers; in fact this led to a brief breakup in 1943. Rodgers then started working with Oscar Hammerstein II, a school friend of Hart.