Robinhood Markets Inc. apparently once again buckled under the weight of a historic trading day, with the online brokerage experiencing a major outage on Monday morning as the S&P 500 traded down more than 5%.

Robinhood via Twitter on Monday morning acknowledged it was down, and said it was investigating the issue. The company's status update page on its website says the issue was identified and the trading platform is back up and running, but the outage caused a wave of user complaints on social media.

The outage is the second for Robinhood this month, with both occurring on days with heavy volume. The app on March 2 went out on a day markets were rebounding after a sharp sell-off, leading to a wave of negative publicity.

Graphic showing stock movement.

Image source: Getty Images.

Robinhood is not alone in having trouble dealing with the high volumes: Charles Schwab had issues as markets plunged. But the stakes are significant for Robinhood, a one-time fintech darling that has seen one of its primary competitive advantages, commission free trading, adopted by larger rivals.