"We owe investors an apology," SSC chairman Tran Van Dung said at a forum Thursday.

It was the first time that any official was apologizing for the inconvenience caused to many investors who were unable to place orders or see the correct data in order to make proper investment decisions.

He was referring to the overload on the main bourse HoSE in recent months. This had happened partly because the market has expanded significantly, Dung said.

"In nearly a quarter of a decade of Vietnam’s stock market, officials like us have always hoped to see the market reaching where it is today in terms of scale, trade value and depth."

But another reason for the overload was the complacency of authorities. The system had been running smoothly for years and they did not anticipate the surge in investment in recent months, Dung said.

One of the biggest challenges for authorities in the last six months was to selecting the best solution among many proposals to have the least impact on investors and to keep the system stable, he said.

One solution was to increase the minimum trading lot from 10 to 100 shares on the main bourse, which reduced the number of transactions by 15-18 percent.

Another was to stop the cancellation or modification of orders which typically accounts for one-third of total transactions.

These moves have helped increase trading value to VND25-30 trillion ($1.1-1.3 billion) in some sessions, he noted.

HoSE CEO Le Hai Tra also accepted responsibility for the overloads.

"As market regulators, when there are issues, we have to partly take responsibility. In the last 21 years, HoSE has always been aiming for the development of Vietnam’s market. That is a commitment and we will always try our best," he said.

For now, a system developed by tech giant FPT is being tested and will be used as a temporary system starting next month while HoSE waits for a South Korea-developed system to be completed by the end of the year.

Despite the overloads, the benchmark VN-Index has been repeatedly climbing to new peaks this year. It is now in the 1,370-point range, up 25 percent from the beginning of the year.