Please make sure that you have your Android SDK set up correctly for 4.12.
- After deleting the folders, did you re-verify your version of the engine?
- Have you also tried opening each blueprint/asset/etc and recompiling/saving them within your project?
- I would also suggest right-clicking in the menu of your Content Browser and fixing up re-directors in folder.
- Have you tried to create a sample project where you implemented ads and received the same error or not?