On Tuesday evening, the NYSE began the rollout of a software release in preparation for the July 11 industry test of the upcoming SIP timestamp requirement. As is standard NYSE practice, the initial release was deployed on one trading unit. As customers began connecting after 7am on Wednesday morning, there were communication issues between customer gateways and the trading unit with the new release. It was determined that the NYSE and NYSE MKT customer gateways were not loaded with the proper configuration compatible with the new release.The "SIP timestamp requirement" mentioned in the statement is an interesting topic in itself. Bloomberg has a bit more detail about this bit of esoterica here: http://www.bloombergview.com/articles/2015-07-09/market-complexity-broke-the-nyse-before-saving-it.