Summary of events is available in English, here is the crucial part copied from the Air Force website:
As soon as the accident aircraft had entered a vertical dive the primary valve in the servocylinder that operates the right-hand stabilator failed; this resulted in differential stabilator deflection, and the flight control system reverted to the backup mechanical mode.
In normal operation inputs from the cockpit mounted flight controls are transmitted to the control surface servocylinders electrically. These inputs are processed by a flight control computer that commands the servocylinders for control surface deflection required for desired aircraft motion.
The backup mechanical mode uses cables and linkages to transmit control stick deflections to the two stabilator servocylinders for continued, albeit significantly degraded, stabilator control authority.
The other flight control surfaces remained under electrical control throughout the accident flight.
Attempts to Restore Normal Flight Control System Operation Failed
The reversion of the flight control system to the mechanical mode caused prolonged tailslide induced oscillations. Recovery from dive cannot be initiated until these oscillations have subsided; once this was achieved, the pilots initiated recovery and attempted to restore the electrical control of the stabilators, but this was precluded by the failure in the servocylinder.
During the recovery attempt, the aircraft failed to respond normally to the application of aft stick while speed was approaching the critical ejection speed. The crew ejected at approximately 875 km/h at the altitude of approximately five kilometers. The aircraft impacted terrain at 1147 hours and was completely destroyed.
The pilots sustained permanent injuries in the high-speed ejection. They are currently on convalescent leave and undergoing a medical rehabilitation program.