Meritz Securities has again come under fire over unstable access to its mobile trading system (MTS).

From around 2:10 a.m. on April 29, posts began appearing on stock-related online communities saying users could not log in to Meritz Securities.

With the timing overlapping U.S. stock trading hours, some investors posted messages saying they could not log in to the app, could not sell and were having difficulty connecting by phone.

After successive access disruptions, investors are strongly protesting what they see as a repetition of IT problems at Meritz Securities.

On April 9, Meritz Securities also experienced login failures and delayed transactions on its home trading system (HTS) and MTS for about 30 minutes after the market close. At the time, the company explained it as a temporary transaction delay caused by a system error.

There were also IT-related controversies last year. In October, shortly after the market opened, access delays and disruptions in processing some quotes occurred on the MTS and HTS. In December, an incident also occurred in which push notifications showing other people's U.S. stock trade executions were sent to some customers.

Meritz Securities has said it would make additional IT infrastructure investment worth a total of 20 billion won to stabilise its overseas stock trading system.

But with unstable access recurring during overseas stock trading hours on the day, system safety is likely to remain under scrutiny for the time being.