It doesn't happen here, please update your system pacman -Syu and run LANG=C sk1

does not work yet, see bt2.txt

Maybe a missing dependency?

So when does this happen, at startup?

As you see - yes.

please execute 'where sk1'

Do you mean 'whereis'? In that case,

[fabio@X200 ~]$ whereis sk1
sk1: /usr/bin/sk1
[fabio@X200 ~]$

as it should be.

problem is in your system color settings

add
print "D"+dark+" L"+light+"\n";
into /usr/lib/python2.7/site-packages/sk1/app/managers/uimanager.py 102th line:

def recalc(self, dark, light, factor):
print "D"+dark+" L"+light+"\n";
r=int(string.atoi(dark[1:3], 0x10)+string.atoi(light[1:3], 0x10))*factor
g=int(string.atoi(dark[3:5], 0x10)+string.atoi(light[3:5], 0x10))*factor
b=int(string.atoi(dark[5:], 0x10)+string.atoi(light[5:], 0x10))*factor
return '#%02X%02X%02X'%(r,g,b)

it should print hex values like:
D#000000 L#ffffff

As root it works as you said, it prints the correct string, but as a normal user it prints:

Drgb:ef/ef/ef Lrgb:00/00/00

and the usual error message. Evidently, my 'dark' and 'light' are RGB values.

edit: I had a look at my .Xdefaults. There I set indeed:

*background: rgb:00/00/00
*foreground: rgb:ef/ef/ef

However, I do not see why sk1 is not compatible with these settings. All other programs work normally.

It looks like sk1 can parse only #?????? format, but not rgb:??/??/??,
please report upstream.