It doesn't matter if you reinstall the application, because the original files will be there in the first place. You need to open the File manager application installed on your phone and then manually locate the place where the GBA Emulator keeps the files. It's possible that some of the files become corrupted, therefore the errors and the chance of succeed when you reinstall. Delete all the files, then reboot the phone, reinstall the original application, place the ROM on the correct path and it should work.