The comments by Christian Sewing, Germany's top banker, come after its financial regulator termed as "unacceptable" the disruptions experienced in Postbank's online offerings, the difficulty clients had in reaching its customer service and in the long processing times.
The issue is a setback for Deutsche Bank in its effort to restore credibility after fines for lapses in money laundering controls. In 2017, the bank was fined $7.2 billion in the United States for its role in the mortgage crisis, nearly toppling the bank.
While Germany's largest bank has apologised earlier for the problems at Postbank, Sewing's comments are the lender's most detailed so far on the matter.
Deutsche began acquiring Postbank, with its millions of clients and roots in the country's postal system, in 2008 during the global financial crisis, but for years struggled to complete its integration.
The bank said in July it had completed a final phase of the integration, but two weeks ago, the German financial regulator BaFin in an unusual rebuke said it had seen "considerable disturbances" at Postbank.
Sewing, speaking at a financial conference in Frankfurt, said that customers had been disappointed "very much".
"We have not lived up to our responsibility here – and now we have to work all the harder to fix the problems quickly and completely - and regain trust," Sewing said.
BaFin has said it would "take relevant supervisory measures if appropriate", and this week the agency's chief Mark Branson further scolded Deutsche Bank's handling of the matter, calling it in a newspaper interview "unacceptable and extraordinary".
Sewing said that Deutsche had made "clear mistakes", underestimating the amount of customer inquiries that would result from the IT integration.
"This situation is anything but good, anything but nice," said Sewing, adding that there had been improvements over the past weeks.
He said that the bank had devoted 400-500 additional workers to the effort and that he is briefed by the board member overseeing the division twice a week.
Deutsche's supervisory board will also examine the issue, Sewing said.
