Third state engine breach was cause of Soyuz/Meridian 5 launch failure

Russia's Federal Space Agency Roscosmos has confirmed that the Soyuz 2-1B Fregat launch failure which, in turn, caused the loss of the Meridian 5 military communications satellite on 23 December 2011, was caused by a breach in the third stage engine. The combustion chamber of the rocket's third stage was reportedly virtually burned through.

Source: FlightGlobal.com