Try removing the splashscreen plugin and reinstalling it.
If that fails, do ionic platform remove android, then add it back. Then do ionic build.
Have you updated you ionic lib or CLI?
The icons and splashscreens should be in the resources folder in the root folder of your project. They copied to the res folder in the platforms/android when you do ionic build android.
Also check the android config.xml to make sure they are listed there.
RGecy