Hiya, I'm getting the same thing here. 1.1.9
In 1.0.0, i get:
An interface already exists under with name: freeplay
stack traceback:
Nauvis-Post-Collapse/control.lua:71: in main chunk
stack traceback:
[C]: in function 'add_interface'
Nauvis-Post-Collapse/control.lua:71: in main chunk
Tried fully removing it, starting a new map in 1.0.0, then backing out to main menu and re-installing the mod and running it from 1.0.0 just to make sure there wasn't any residual problem files, and no luck :(