Error 1: I tried importing an old FARL blueprint from 1.0 with the now legacy rails in it and it gave me an error: Error: FARLS/FARL.lua:2396:attempt to index local 'altRail' (a boolean value).

Error 2: Upon using the 'Read' button to import a diagonal rail blueprint for the farl to use (using the now standard rails) it adjusts the blueprint. It moves the signals and power pole up 2 tiles. Not sure why it's doing this when it should just be reading the contents and not editing it for this functionality.

Error 3: While FARLing it adds/removes concrete of its own accord regardless of if 'Place Concrete' is checked or not. After getting out of the FARL and back in it seemed to work again so this one may be hard to reproduce. For example: While moving in a straight line, it was putting down my blueprint, but it was also removing the existing concrete.

Error 4: This may be related to Error 2. While FARLing and changing direction, it stops making other rails in the diagonal direction. And throws an unexpected error when straightening out (either continuing the turn to the 90 or returning to the same direction).
The error in question:
2772.041 Script @FARLS/lib_control.lua:23: 29327167 Unexpected error: FARLS/FARL.lua:2758: attempt to perform arithmetic on field '?' (a nil value)

I will say that I was playing on my SE test map, so I'm not sure if that has anything to do with anything.
I've also created an issue on your github page so that I can upload screenshots and provide the save file.
https://github.com/shoter/FARLS/issues/1