Just a guess, I'd say your game setup is home planet value of good, which means the game is trying to create a home world for you that is a large methane rock world, which does not exsist in your mod. Either use average planet value, or change your race to gas giant/hydrogen or none/ice and it should work.
If you are not selecting home planet value good, I need more information. Would probably need to see the files.
EDIT: DANG! Fourth? I gotta type faster.
[ March 12, 2004, 17:55: Message edited by: geoschmo ]