This problem is quite common when you've tried to load a shader that isn't compatible with your game version, or when you've placed a custom shader inside the Sideloader Modpack folders, corrupting how the mods in that pack are loaded. I don't have extensive knowledge, but simply put, and as it was explained to me a long time ago, you shouldn't put anything else inside the Sideloader Modpack folders. When the game starts, it loads those folders first, and then the regular mods.
I advise you to delete the cache folder located inside the BepInex folder. I'd also suggest you manipulate the config (the .cfg files), although I'm not sure if KKUTS or other shaders generate this file where you can manipulate their configuration using Notepad as a bridge. In the worst-case scenario, you restart the plugin from scratch.
Basically, all of this happens because the internal registry gets corrupted; it has little or nothing to do with zipmod itself, although I suggest you check if the shaders you want to use are for Koikatsu or Koikatsu Sunshine.