This was the sixth attempt at a robotic circumlunar flight in support of the L1 piloted lunar program and the first after the resounding success of the American Apollo 7 in Dec. 1968.
The Proton launch vehicle lifted off on time and first stage operation was nominal. However, during second stage firing, one of the four engines of the stage spuriously switched off at T+313.66 seconds, about 25 seconds early.
The other engines continued firing (and could have actually compensated for the loss of thrust), but subsequently, the primary third stage engine also switched off early during its firing sequence, at T+500.03 seconds, due to a breakdown in the main pipeline feeding fuel to the fuel gas generator.
After a near-ballistic flight, the L1 payload landed (and was recovered) southeast of Irkutsk near the border between the USSR and Mongolia.
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.