D2L Brightspace - Why the LockDown Browser Dashboard is Not Opening for One Course
Posted by Respondus Support

If the LockDown Browser Dashboard is successfully loading for many courses, but failing for one, the following are possible causes: 

1. There may be an empty or corrupted value in the default "Submissions" message. Here is a KB article that deals with a related symptom of this:  http://support.respondus.com/support/index.php?/Knowledgebase/Article/View/223/19/not-all-my-quizzes-are-showing-up-in-the-lockdown-browser-dashboard

2. The Self-assessments or Survey tools are disabled in the specific course. These are required (along with the relevant instructor permissions for them) for the Dashboard to open. To check on this, go to Course Admin > Tools and make sure both of these tools have a check mark showing they are enabled.

3. One or more of the quizzes has an IP restriction.  The D2L SOAP API which the Dashboard uses does not permit this. You'll need to remove the IP restriction from all quizzes in the course before the Dashboard will successfully load.  

Note: if the IP Restriction icon remains on the quiz (even after you've deleted the restriction from the quiz's "restrictions" tab) the Dashboard may still fail to load. If this occurs, a workaround is to delete the quizzes with the restriction, then create new ones. Otherwise, you will have to contact D2L to remove the icon.

4. The Instructor has invalid role permissions. Please refer to the LockDown Browser Administrator Guide, under the section "Instructor Roles and Permissions for IMS Configuration."

5. Invalid or special characters or symbols in the course title or quiz titles (such as colon, hash, ampersand, emoji's, etc.)

6.  There may be too many quizzes in the course. Though the LockDown Browser Dashboard LTI tool does not place a limit on the number of quizzes in a course, the D2L SOAP APIs might not be able to handle a large number.

Any quiz limits are a function of the older D2L SOAP APIs and their ability to handle large amounts of data. It all depends on the amount of memory on the D2L server that's been allocated to the SOAP APIs
It's quite possible those D2L API's cannot handle the number of quizzes in the course experiencing the problem.  As a troubleshooting step, you might try cutting the course in half to see if that resolves the problem.

7. If you are a D2L administrator, and the STOP sign error is occurring for some courses, you should first try impersonating an instructor role....and then try opening the Dashboard again.
If the problem persists, add yourself as an instructor in the course and try again. If things then work, this means there is likely a permission missing from your admin role that needs to be adjusted.