If you start a ProctorTrack based LumiNUS quiz and the ProctorTrack app closes during the quiz and the quiz gets submitted then please follow the below steps to resolve the issue:
This issue could be possibly due to VPN or Antivirus running on the system. Therefore, disable them and then retry to take the quiz.
If the issue still persists, then you may try to configure the network settings to use Google public DNS. For more details on this refer to: https://developers.google.com/speed/public-dns/docs/using