ava.lang.IllegalArgumentException: Slot 241 is already occupied by codechicken.enderstorage.BlockEnderChest@3ef88e3e when adding ic2.common.BlockRubSapling@3042a8be
this line is your issue
you are useing enderchests on your server
to fix this goto your config folder
open the EnderStorage.cfg
change the block ID.
try 178 I am useing this on my server and it works fine
Slot (A Number) is already occupied by
When ever you get a look at the 2 mods involved in this case Enderstorage and IC2
you could also just change the item in IC2 BlockRubSapling from 241 to say 251
the ids you need to find ones not in use. that all depends on the mods you have installed