+1 to MrCervelo's issue, I'm having the same problem. Also of note is the fact that the file is present and seems equivalent the others in the directory.
EDIT: Deleting the agricraft configs worked for me, though this doesn't seem like an ideal solution
Slow, Captainnana, et al: you may wish to add a clause to your terms, etc stating that each playthrough must use only one player. At the moment it's technically allowed to play this multiplayer, so far as I can tell.