If you look at the Steam player charts for the game you'll see when it's working vs. when it's not. Off-peak it works fine, but right now the player base is ramming up against their temporary player cap for hours at a time on-peak. If you try to connect when there are thousands of others also trying to connect, that's when things go south. That was the case for much of the weekend.

Edit: Here's a chart illustrating what I mean:

In the last 48 hours, player counts hit 400k at about 7pm ~~Eastern~~ UTC and just stayed there for 6-ish hours. That isn't normal, almost all other player count charts show a gradual rise and fall over the course of a day. The devs implemented an artificial cap after they found that their servers bog down when there are too many active players, basically sacrificing the peak time login experience to preserve the in-game experience. If you try to connect while the active player count is pegged, you're essentially joining a swarm of other players who are also trying to connect at the same time. That swarm is likely DOSing some aspect of their own login systems.