This is a known issue in the upgraded version 3.5-200006 of the V4S Kiosk application. If you add a walk-in volunteer from the Job or Shift details screen and after adding all the volunteer information and click the Signup button, the volunteer is actually added to the Job/Shift. But, the screen on the device shows the loading spinner and that does not stop.
We will fix this bug in the future releases of the V4S Kiosk application.
As a workaround, you can click the back button and go back to the Job/Shift screen and the volunteers can continue to use the app regularly.