Complex sentence: He was prevented from encouraging the soprano Kirsten Flagstad from retirement, or signing the conductor Otto Klemperer--a misjudgment, as Culshaw noted in his memoirs, that Walter Legge of EMI was quick to remedy, with great artistic and commercial success.
Split into simpler sentences: He was prevented from encouraging the soprano Kirsten Flagstad from retirement, or from signing the conductor Otto Klemperer. The latter misjudgment, as Culshaw noted in his memoirs, was not repeated by Walter Legge of EMI, who signed Klemperer up with great artistic and commercial success.