It seems like steam is periodically rejecting some users and failing to authenticate them.
We’ve reset the official servers and this seems to have fixed the issue for 99% of people. If you’re having trouble connecting to a community server, you’ll need to wait for them to restart their server. For the few who are having trouble connecting to the official servers we are looking into this issue, however it appears to only affect a very small portion of users. It is recommended you apply any pending steam updates and try again. We will keep you posted with any new developments.
EDIT: This seems to be happening again to a bunch of users, we changed nothing so it has to be an issue with steam itself, or we’re authenticating users wrong. I have contacted the person responsible for maintaining this element of the game and they will look into it as soon as possible!