Steps to reproduce:

1. Create Course with one trainer and two students
2. Create a Scheduler instance and one future standard time slot (15 min)
3. Student 1 books the slot
4. Trainer revokes the appointment -> Result: participant has been removed from the slot, slot is available again
5. Student 2 books the same slot that is now again free for booking -> Result: Looks like slot has been booked correctly from student perspective
6. Trainer view does reflect the new booking, slot is empty, should show the student
7. Back on the student view, a click on "Edit booking" results in an error message "Can not find data record in database table scheduler_appointment."