TOKYO, Aug 20 (Reuters) - The main banking unit of Mizuho Financial Group (8411.T) said on Friday its branches throughout Japan were unable to process transactions, the latest in what has been a long, high-profile series of embarrassing system errors for the bank. A report in June commissioned by the bank found that its corporate culture - including an atmosphere where managers are reluctant to express their opinions and unable to respond well to crises - was to blame for its tech problems. Japan"s third-largest lender suffered four system breakdowns between February and March this year, despite having spent more than $3.6 billion to overhaul its systems in 2019. That revamp followed two large-scale breakdowns in 2002 and 2011. A Mizuho official said the glitch is also affecting the group"s trust bank unit, and the cause of the latest problem appears different from the previous ones. The bank said its automated teller machines (ATMs) and online banking services are operating. Reporting by Makiko Yamazaki and Yuki Nitta; Editing by Stephen Coates Our Standards: The Thomson Reuters Trust Principles.