Either two things. The GuidAuth server is getting swamped with requests and it doesn’t get back to your client in time, or your cdkey has become messed up.
For option one, the best thing you can do is disconnect and reconnect again. If it keeps coming up, try some of the stuff for option two.
If your key has become corrupted, you need to have ET generate a new one. You can do this by going into the server browser and turning off PB. Close the game and restart. Now, go back into the server browser and turn on PB. This will generate a new key for you, and you should be able to connect just fine.
RK