Started using a Lumia Icon yesterday and had this exact same problem, had no idea it was "a thing" - I just kept trying to uninstall/reinstall, soft reset and had the same result of Error #602. Messenger still works on my 1020, but then again, I'm not installing Messenger 'fresh' on that (it's been on there ages). It has to be a server side issue and we'll just have to wait for a fix. Annoying though. If it's any help, remember you can always go to m.facebook.com in the browser and get to your Messages that way (that's what I'm doing for now).