This afternoon Rainbow Six: Siege was supposed to be launching its beta but there’s been problems that have delayed the launch,
The main issue appears to be disconnection and matchmaking problems according to Ubi and therefore they have “postponed” the beta “until a later time”.  The problem was detailed by Ubi in their support thread on the matter:
Game sessions are not starting properly and we’ve encountered server issues, leading to failed connections. We’ve identified the root issue that was introduced after the Technical Test while fixing a separate aspect of the game. A fix is currently under test and will be deployed within the next few hours.
There’s no exact time/date for the launch of the beta although the latest update suggests we could see some action at around 6PM EST. Regular updates have been appearing on their Twitter feed throughout the day so if you’re waiting to take part then keep an eye on that and the support thread.
Usually with betas so close to launch they run relatively smoothly and can be considered a marketing stunt but it looks like Ubi actually did need to test their systems more. If you were thinking about picking this up on launch day it might be a good idea just to hang tight and see what happens when the game launches.
As soon it goes live we’ll let you know.
Update 10PM EST: Ubi has now given another tentative launch time.
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 (Thursday) 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.
Published: Nov 25, 2015 09:01 pm