The NYSE said in a statement Thursday that the upgrade caused communication problems between its trading software and other systems, causing customers' orders to not go through.
The problem became severe enough by late morning Wednesday that the exchange decided to suspend trading at 11:32 a.m. Eastern time so the problem could be addressed. Orders were cancelled, trading software was restarted and backup units were brought online, the exchange said.
The exchange restarted trading at 3:10 p.m. Eastern.
Wednesday's shutdown was a black eye for the NYSE, but ultimately it did not broadly impact Wednesday's trading. The Nasdaq still operated normally and several other exchanges were also available for investors to execute trades while the NYSE worked through its problems. While the exchange was only open for 50 minutes in the afternoon, it was enough time for investors to close out their positions for the day.
The outage came at a time when traders had plenty of other things to worry about. There have been concerns about China's plunging stock market and a logjam in talks between Greece and its creditors that has weighed on investors' mood all week.