to reproduce the bug:
1. as teacher, add a calendar block
2. now logoff and login as student enrolled in the above course
3. as student, try to add a user event
4. Moodle allows student to edit a new user event with all the needed parameters
5. now clicking "save", Moodle returns an error "you are not allowed to do this", due to lack of student's capability.
If the student does not have such clendar capability by default, the New event button should not be available. Otherwise, enable this capability by default in the student legacy role.