Cosmic Rebirth updated to version 2.1
An issue with build 281 is that bosses on Mars and the Moon will fail to spawn and therefore completely stop the player's advancement through the game.
Type in the modpack name (Cosmic Rebirth) or paste the following url into the search box.
Finally, click Install at the bottom right of the launcher after you select Cosmic Rebirth from the list on the left. The launcher will handle everything else!