This happens when you join someone on Fabled or Mythic difficulty for missions/Kepler patrol. Can you try changing your world tier down to Legendary or Brave by using the World Tier icon to the right of Caldera? Its the skull icon. if that doesn't work can have someone invite you to a fireteam, and have them use the World Tier node to launch you into Kepler? I can invite you if necessary. Either of these should flip your World Tier back down and let you back in - done it a few times to help people out with that problem.