The mobile version of the game currently contains a bug that causes the unlocked expansions and their prices of one world to be displayed on another world. This can, in some cases, allow you to attempt to place an expansion you have already placed or not yet unlocked, which leads to the game displaying an internal error. This error prevents the game from deducting resources from you or reducing the number of placeable expansions, so you won't loose anything due to this, however we realize that this can also cause some confusion, as expansions seemingly disappear that previously have been available. The browser version of the game is not affected by this and can be used normally. For the mobile Version, we recommend not making use of the "Switch Worlds" function for now but to log out of the game and then to log back into the desired world manually. We also have a fix planned for the next version of the app, 1.136, which will release next week. Thank you for your understanding and patience!