FUUUUUCK we should've thought of this *sob*
The problem is that your filepath is too long. We built FSO to work in C:\Games\FreeSpace2, not in C:\Program Files (x86)\Steam\steamapps\common\Freespace 2\
When it tries to load in files in the latter directory, the path is too long and it freaks out - there are more than 128 characters in the path + filename.
Can you move the Steam install directory using Steam Mover? Move it to a shorter location, like C:\Games\FreeSpace2\, and everything should be fine...