QTI grading tool: Upload leads to error message

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")