I had this problem with not being able to log in with the steam client but now figured it out or a loophole if you will, to those who play steam client of SK, restart it.

The way I did was Ctrl+Alt+Del>Task Manager>Processes>look for steam>end process>start it up again and try logging in SK with steam and it should work.

Another way is Restarting steam and letting it update

If those 2 ways do not work, the first one should at least work as it did with mine, if not, then I don't know

Hope this helped :P