It is 100% a sky issue. I've had the same issue since last year! Deleted the app several times, I've left it about 6 months now because I thought it was just broken. downloaded it again and I still cannot get past the sign in. It accepts my log in, says sorry there's a technical issue then takes me back to the sign in page. So it's impossible for me to sign in and sign out, I've deleted it several times over the last year or so. Obviously my phone has been switched off and on again. And absolutely no change in the app. So unless this comment saying that the problem isn't with Sky has any solutions, then they should keep their unhelpful comments to themselves.
