- Open the Settings app.
- Scroll down and tap on iTunes & App Store.
- Tap on your Apple ID (an email address) and tap Sign Out (note that you will need your password in step 6 to sign back in).
- Turn off Wifi and cellular connectivity by enabling Airplane Mode.
- Reboot your device as described here. Note that this procedure is NOT the same as merely powering your device off and on.
- Disable Airplane Mode.
- Head to Settings > iTunes & App Store and sign in with your Apple ID.
- See if the download of the app resumes correctly. If not, open the App Store, search for the app in question and tap on the stop button. Wait a couple of minutes and then tap on it again to resume the download.