i didn't had the newest game version, if anyone is having the same problem

Is it a 0.14 save or was it already loaded in 0.15? Is FAT alreay in the save (which version) or first time trying to add Fat controller?
Loading the save without Fat enabled, saving and reloading with Fat enabled should load the save.

The strange thing is that LuaTrain has the key fluid_wagons ...

after your newest update i'm still getting this error message.
this is when i load a save game. so i can't play with fat controller on right now, which is really sad.

Error while running the on_configuration_changed: LuaTrain doesn't contain key fluid_wagons.
stack traceback:
__TheFatController ...