This happens when you have copies of the pre-September-Steam-patch shaders installed. I don't believe 2.1 actually included shader files in the first place, so it's likely in your actual default directory.
Go into your Corruption\Data\Art folder and delete the folder named Shaders.
IF that folder doesn't exist, you can check Corruption\Mods\Imperial_Civil_War\Data\Art\ and delete shaders from there, if it exists.