Looks like some weird issue with universal skins introduced by BeardLib.

I don't really have an idea how they work, unfortunately. And I don't see a way to quickly fix it without researching a lot around those mods.

Maybe a skin is broken? Because as far as I see, SDSS checks if it is a universal skin and then checks for the ID of the skin. One of your skins seems to be a universal skin but has no ID, which is weird.