To reproduce the issue:
- Create a new course
- Add the calendar course element
- Add an event in the calendar
- Click the feed icon
- Click again the calendar course element in the course menu left
-> Calendar disappears because the feed icon set per default the configuration of the user for this calendar to invisible and not aggregated. But otherwise the default setting are visible and aggregated (if there are not configuration saved on the database for the user for example), which make a discrepancy between what the user see and what is saved on the database.