When you try to import material downloaded from Quixel, 3DCoat it is expecting a zip file, however the new downloaded Quixel material is not a zip file.
Now inside 3DCoat (from version 2023.40+), you can add the additional Quixel folder by following these steps:
1. Open 3DCoat.
2. Go to the Edit > Preferences.
3. Navigate to `Input/Output` tab.
4. Here, you should see an option for `Additional Quixel folder’
5. Set the path to where you’re Quixel folder materials are and hit apply and close.
Restart 3DCoat and you will be welcomed with the following: