Course Statistics: Quarter view results in internal server errors.

[simonbohnen]

Describe the bug

When clicking on "Quarter" in the course statistics, several server errors are thrown and the UI does not update.

To Reproduce

1. Go to Course Management, select a course, and click on 'Statistics'.
   (I selected a course with many participations, e.g. ITP)
2. Click on the 'Quarter' time range button
3. Verify that Internal Server Error warnings are shown in the top right and that the UI does not update.

Expected behavior

The Quarter view should be displayed without any errors.

Screenshots

Which version of Artemis are you seeing the problem on?

7.9.1

What browsers are you seeing the problem on?

Chrome

Additional context

No response

Relevant log output