An incompatibility with a software update and subsequent attempt to fix it were the root cause of glitch that forced a nearly four hour long suspension of trading on the New York Stock Exchange on Wednesday.
The exchange traced the problems back to an update applied to a single system on Tuesday evening. The new software was related to an upcoming industry-wide test of a new timestamp procedure for communications.
On Wednesday morning as customers began connecting to the system, communications issues arose because of an incompatibility between the new software and configurations in customer systems. The software in the customer systems was updated prior to the market open at 9:30 a.m. EDT, but that update caused additional issues, the NYSE said in a statement.
The issue was investigated shortly after 11 a.m. and within 30 minutes, the exchange took the decision to suspend trading.
It remained suspended until just after 3 p.m. EDT, when the market restarted for the final hour of trading.
The NYSE accomplished that by switching over to back-up servers at a Mahwah, New Jersey data center that were not loaded with the new software.
The suspension hit the headlines around the world because it was one of the biggest disruptions faced by the NYSE in years, but its actual impact was muted because NYSE-listed stocks continued to trade on other exchanges.
It came hours after a glitch grounded hundreds of United Airlines flights across the country, but the NYSE and White House moved quickly to dispel fears that the outage was caused by a cyberattack. United Airlines later said its problems were traced to a faulty network router.