This is a known issue only in iPhones and iPads and will be fixed soon.
If you enter the wrong App Key along with your ID and password, when you click on the Login button you will see an Alert message that says "App Key Error Invalid App Key". When you click OK on this message, you see a loading sign and which does not stop and does not let you re-login to V4S Kiosk.
If you see the loading sign that does not go away, you should uninstall the app and reinstall it before you can login to V4S Kiosk with the correct App Key, ID and password..
You will not see this issue on Android phones, as you can use the back button to go back to the previous screen. And then login to V4S Kiosk using the correct App Key, ID and PW.