This forum post explains that Ubisoft is postponing today's planned launch of open beta testing in Rainbow Six: Siege. They offer a brief FAQ detailing the matchmaking issue they are fixing on all three platforms, and in the meantime, the closed beta continues on. Here's the latest update on where things stand:

As communicated with you today, we have performed a maintenance on XB1, PS4 and PC this afternoon. Some platforms are still under maintenance but we are working to complete them as quickly as we can. We will be monitoring how the fixes impact the game experience throughout the night. We will give you another update tomorrow at 10AM EST. In the meantime, our Closed Beta continues. Please give us your feedback now that the update has happened and tell us how the experience is for you. Thank you for your patience and cooperation.