Alright, what am I not seeing.

I see this error, which refers to bevels, even though I keep turning
bevels off

ERROR: Assertion '((abs(bevel_size1) + abs(bevel_size2)) < len)' failed:
"Combined bevel size exceeds length of screw" in file , line 1800

include <BOSL2/std.scad>
include <BOSL2/threading.scad>

eps = 0.01;
slop = 0.2;
$fa = 3;
$fs = 0.2;
inches = 25.4;

ht = 15;
id = 114.5;
wt = 1;            // wall thickness
tp = 3.5;        // thread pitch
td = 1.2;        // thread depth
th = 1.9;        // thread height

threaded_nut(nutwidth = id + 20, id = id - 2*th, h = ht, bevel = false,
shape = "square", bevel1 = false, bevel2 = false, pitch = tp, starts =
4, $fa=1, $fs=1);

--
This email has been checked for viruses by AVG antivirus software.
www.avg.com

Since it seemed to be complaining about the thickness of the nut, I
increased it until it stopped complaining.

The resulting picture tells much of the story, I think:

Note the huge bevels at the top and bottom.

That module has "ibevel" to control whether there is inside beveling,
and the default is true.  But I don't immediately see what controls how
much it bevels.

Random other note:  normally you shouldn't need to specify $fs and $fa
on an individual call.  Just set them to something reasonable at the top
of the file - I like $fa=1 and $fs=1 because it's easy.