The new Test Exception feature allows instructors to establish separate availability dates for individual students or groups of students. This is useful for students who may be traveling or ill during a regular exam window. Setting the availability dates should allow only the student or students included in the exception to access the test after the availability window closes. However, if a Due Date is specified and the setting for "Do not allow students to start the Test if the due date has passed" is selected, students with availability windows after the Due Date will be prevented from starting the test.
Resolution
To avoid this, simply uncheck the option for "Do not allow students to start the Test if the due date has passed" and use the global availability dates for the Test to restrict students from accessing the Test. Then, the availability exception will allow the student or students to access the Test as normal and begin the Test. These submissions will be marked as Late by Blackboard, but that warning can be ignored.