~LIGHT5F 255 255 255 17000
That is one of the light textures i use. Its in the half-life wad. You can see that all of the characters, including the ~, are included.
Also, i beleive its case sensitive, make sure the texture in the lights.rad, has capitlized letters, and lowercase letters, according to the texture name. I remember almost all of the halflife light textures being all caps, like the one i used above
edit: why dont you post the lights.rad line for the texture thats not working, i might be able to figure out the problem. just copy and paste like i did at the beginning of this reply