Edit -- Update on the update
We have pinned it down, for some reason it's not build specific, for some players it is complaining you are not logged into Steam, usually it will ask you to log into Steam but for some reason it's not triggering that. Annoyingly we have tried logging out of Steam and running the game and it still works for us! But we are looking a fix that avoids that complaint
We just wanted to apologise to the players who cannot play the game after Update 0.26.4.0. It is strange as it is working fine for us and many players. We are just trying to pin it down. My one theory is something has gone wrong in the transfer from the branch we were testing on to the main branch. We are on it and will have a hotfix out in the next few hours. Again I am very sorry this happened, we have played the update for days without problems so were shocked to hear that some players couldn't even get the build running.