Lots of great Test Setting and Troubleshooting Tips can be found here. There are some good tips for this issue unfortunately experienced by all test givers!
““Help! I got kicked out of my test…” Symptom: The student is kicked out of an assessment when the submit button is pressed. Cause: The java session has been terminated due to inactivity, so the submission is unsuccessful. Almost always this issue occurs with randomized tests or where certain questions are randomized.”