That's probably an error on the hosts side. Remember, you can only join the games that have a brown door icon (waits for people to join) on the right, not a clock (running). Golden door means, only registered players can join, a lock says that the game is password-protected and you can only join if the hosts gives you the password through other means than the lobby selection. Red door means that people are allowed to join even while the game is running, but that needs a really long time and works only in ~5-20% of the cases, so it's usually more of a symbol that the host hasnt played Clonk for a long time yet ;) (hasnt heard about the red door option, which is enabled by default).