Nuclear safety experts found failures in the emergency cooling system at Brunsbüttel plant, an official in the northern state of Schleswig-Holstein said.
The plant was disconnected from the main electricity network after it became overloaded on June 28, although it continued to produce power for its own use.
Hours after the incident, a fire broke out at the nearby Krummel nuclear plant, which also owned by Vattenfall.
German officials accused Vattenfall of failing to report the full extent of the problems at its plants.
The firm denied a cover-up, but the scandal prompted the head of its Europe branch to quit on Wednesday, and caused an outcry in Germany, which is deeply divided over whether it should be using nuclear energy at all.
The country has begun a long-term phase-out of its nuclear energy programme and expects to mothball the last of its 17 plants around 2020.