If you are seeing a blank screen, there's a very good chance you are in a country where Google is blocked. Vellum servers run on the Google Cloud platform which is blocked in China, Crimea, Cuba, Iran, North Korea, Sudan, and Syria. There are certain instances that will trigger a block, and subsequently, deliver a black screen in Vellum. Usually, our customers in China will not have issues with the Vellum App but every once in a while, this will happen.
We recommend using a VPN on your device to access the wallpapers. We have users who have had success with ExpressVPN. There's a tutorial on how to change the app store country to download ExpressVPN in China. https://www.expressvpn.com/support/troubleshooting/change-app-store-country/