TSB has apologised to customers who faced disruption to their online and mobile banking over the weekend.
The bank did not say how many customers were affected, but 1.9 million use its digital banking services.
TSB is still recovering from an IT fiasco in April, when customers were left without access to online banking services for several weeks.
The bank said that its systems seemed to have returned to normal by mid-morning on Monday.
A spokesperson added: "We continue to monitor the situation and will continue to update our customers."
After planned maintenance on Friday and Saturday, TSB users reported that they were having trouble accessing their banking services on Sunday.
The bank tweeted on Monday morning that some customers were facing problems.
We’re aware that some customers are having issues this morning using Internet Banking, the mobile app and telephone self service. We are currently working to fix this issue. Please accept our apologies for any inconvenience caused.— TSB (@TSB) September 3, 2018
Consumer body Which? said this was a "real blow" to consumers who had stuck with the bank since April's meltdown.
The IT problem in April was caused when customer data was migrated from an IT system operated by Lloyds Banking Group, which used to own the bank, to a new one one managed by its new Spanish owner, Sabadell.
Gareth Shaw, Which? money expert, said: "Customers can incur fines, penalties and fees when they're not able to access their finances, so the bank must offer compensation to all those affected.
"For TSB customers at their wits' end with the bank - there has never been an easier time to switch, and the current account switch Service makes the process as painless as possible," he added.
In July TSB said the IT meltdown had cost £176.4m and pushed it to a half-year loss. At the time it said about 26,000 customers closed their TSB account in the second quarter of the year but that more than 20,000 customers opened a new bank account or switched their account to TSB in the same period.
The bank has about five million customers.