I've been playing for a while without any problems whatsoever. Suddenly 2 days ago I started getting instant disconnects on log-in. With a 2nd account the problem also happens. I've made an exception to my firewall for the program, I've disabled anti-virus, redownloaded the game, deleted WTF and Cache folders. Nothing worked.
I haven't made any changes to my pc set-up or internet connection, so I can't explain what's causing this issue after a long period of playing without problems.