Complaints by bank customers to Germany's financial regulator nearly doubled last year, figures published on Monday showed, with many of them directed at a single lender.
The regulator, BaFin, said that overall complaints by bank customers rose by 87% to more than 27,000 last year.
Although BaFin did not name the lender, last year it publicly criticised Deutsche Bank for its botched integration of Postbank, which left customers complaining that they were locked out of their accounts and unable to reach call centres.
BaFin installed a special monitor to oversee how Deutsche addressed the customer service issues.
"A considerable portion of complaints about disruptions in customer service at banks were linked to one financial institution," said Christian Bock, head of BaFin's consumer protection division.
The issue has been a setback for Deutsche Bank's effort to restore credibility after fines over the last decade for lapses in money-laundering controls and other penalties.
Deutsche Bank began acquiring Postbank, which had millions of clients and roots in Germany's postal system, in 2008 during the global financial crisis, but for years struggled to complete its integration.
Germany's largest bank said in July it had completed a final phase of integration, but in September BaFin in an unusual rebuke said it had seen "considerable disturbances" at Postbank.
Several members of Deutsche Bank's management board - including Chief Executive Christian Sewing - got a small cut in their bonuses as a result of the glitches.
