Mizuho Bank said Thursday a widespread system problem that affected its automated teller machines over the weekend was caused by a process related to the switch of customers' data to digital passbooks that began in January.

The bank had planned to transfer deposit accounts that have been inactive for a year or longer to the digital passbooks in stages, but is considering postponing the process due to the problem.

The issue, which became apparent on Sunday, resulted in over 4,300 ATMs failing to function and keeping 5,244 bank cards and account booklets inserted by customers. All of the affected ATMs were back in operation by 3 p.m. Monday.

The banking arm of Mizuho Financial Group Inc. had earlier explained its systems were overwhelmed when updating the status of fixed deposit accounts and processing other transactions.

Another system failure caused 28 Mizuho Bank ATMs in areas including Tokyo and Osaka to stop working late Wednesday night, with 29 bank cards and passbooks not returned to their owners by the machines.

Forty percent of the cards and passbooks have not yet been returned and the bank said the cause of Wednesday's problem was different from Sunday's outage.

Mizuho Bank in January started offering digital passbooks which allow customers to access account information and activity online while charging those who open a new account 1,100 yen ($10) if they request a paper booklet.

Related coverage:

Japan's Mizuho Bank reports large-scale ATM system failure

Japan's Mizuho Bank restores ATM services after system failure

Mizuho Bank ordered to report preventive steps over major ATM glitch