"We’re currently seeing issues with matchmaking times and disconnection on all platforms," Ubisoft wrote on the Rainbow Six forums. "As such we’re postponing the Open Beta until a later time and keeping the game under Closed Beta status. We apologize for the delay and can assure you this is our top priority."
No indication was given as to whether this could impact the scheduled release date, which is set for December 1. I've reached out to Ubisoft for more information and will update when I receive a reply.
Update: Ubisoft says the postponement of the open beta will have "no impact whatsoever" on the release of the game.