If you are running Civil 3D 2018 with the .3 update installed, and you are receiving a fatal error while creating a best fit alignment, then the solution, according to Autodesk, it to UNinstall the .3 update.
Here is the tech bulletin on this, and an image of it below (captured on 04 SEP 2019) for reference, in case Autodesk renames, moves, or removes it.
