I've already played a few Co-op games yesterday and today without problems. When I tried to start the next game, this error came up, and since then I can't download any new files, including vs. multiplayer.
Since then I updated the client, cleared the game files and cache data, then made a clean re-install and again cleared game files and cache. No antivirus is or has been running, and only the windows 10 firewall is. Forever.log from after the re-install is attached.