@not_Frozer please make alls sure to log out and back in one time before trying again. You might also don´t want to change the settings while everyone is already in the custom game lobby. make the game settings, then invite everyone else to the game.
This is not necessarily a bug. Sometimes players will decide they don't want to trade with you specifically. This can be because you are ahead on points, competing for a resource intersection, are about to get longest road, etc.
AI opponents will very rarely accept a trade with a real player.