Did you click on the Faculty Information and Updates link on My Courses Page in BB?
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.”