How to reproduce:
- Coaching Tool --> Grading assignments --> Tab Grading assignments
- Select a grader and click below either the button "Assign grader" or "Change grader"
- In the dialog search for and choose another user
- In the step "Contact" try to attach a file and upload it
NOK: Error message appears.
OK: File is uploaded and attached. (For comparison: It works when using the button "Send email")