You should have started with that :)
This is definitely not a problem with your client then.
So, the exact name of the hat is Warrior Helmet, right? Also, please tell me race and gender of your character.
If you can't see one of our custom textures, this means some files in your game client are corrupted or absent. Please run the full check via Averia Updater.
This. Though I am not sure it's working as intended by Koreans. Anyway, I reported that to technical admin, and if it's still a bug, it will be fixed.
Sorry for leaving you without an answer for so long.