Missing sprites/textures? Magenta elements

I had the same thing yesterday when i accidentally changed the shader on the default material. It may be a pain, but I'd suggest creating a new material specifically for the UI elements you have, with a mobile shader attached. This should resolve your issue as the new material/shader should be bundled into the build. I fixed my default shader in c# by changing GameObject.renderer.material.shader="Standard"; but I would imagine it's better practise to have your own material.