Afaik its the UI-Quickslot.blp controlling the actionbar borders. But maybe I am missing another texture file. I'm not sure.
Anyone know what could cause this or a possible fix?
I am not very good at this but, I have read textures are handled differently now (the default ones) since 8.0.1 - the game could very well be seeking the new texture and finding it perhaps due to outdated code?