Here's my theory: The people who cannot sign into their account, has not updated their version of Destiny. Or rather, Bungie hasn't updated their version. I did a fresh install of Destiny today to try and see if that fixed the login problems, but it didn't. However, when checking the game files on my PS4, I saw that the version I had – the one I just had downloaded and updated via PSN – was version 1.0.1.3 Which really does not compute, seeing as version 1.0.1.4 and 1.0.1.5 are already released.