I was able to play earlier today, but have been unable to log in since about half an hour ago.
Clicking on the red Login button leads to a long “authenticating…” popup and then an error message that the game “cannot connect to the login server, check connection or firewall and try again.” Nothing has changed in my connection or firewall in the past couple of hours. Restarting launcher or steam, or clearing steam cache have not made any difference.