I find that when SK check-in on their glitchy App fails, the glitchy website usually does the trick and when you complete the check-in there the App usually updates. I have only had cases where SK insisted on doc checks before giving me a boarding card where I didn't get one issued. In return that could conflict with routes where other airlines on the same route would or would not give me a boarding card where SK would or would not. There is incredibly little consistency, most issues where on flights to LHR and ZRH.