Progress M-27M Failure Caused by "Design Peculiarity", Space Policy Online
'Russia's space agency Roscosmos today revealed the results of its investigation into the April 28, 2015 Progress M-27M cargo ship launch failure. A "design peculiarity" related to frequency-dynamic characteristics between the Soyuz-2.1a rocket's third stage and the Progress spacecraft was to blame, it said."
Proton-M Failure Due to Same Design Flaw that Doomed 1988 Mission, Space Policy Online
"Russian authorities investigating the May 16, 2015 Proton-M rocket failure have determined that the root cause was a design flaw in the turbopump for the third stage steering engine and concluded it was the same root cause of a prior crash in 1988. Igor Komarov, head of Russia's Roscosmos space agency, said today that the Proton-M third stage failed on May 16 because of excessive vibration in the turbopump."