Clearing the calendar storage:
During this process EVERY EVENT within your local database is erased. So any unsynced content (including items within local calendars) will disappear irretrievably. To keep these events it is important to sync them or to create local backups (i.e. by exporting with our application).
1. Please open the Android system settings of your device and go to → Apps or → Applications
2. Choose the filter → All at the top, scroll down to → Calendar Storage and tap it
3. Hit the button → Clear Data and the calendar storage is cleared.
Once again please make sure that you created BACKUPS of your local data since ALL EVENTS on your device, that have not been synced, will be deleted irretrievably.
After erasing the calendar storage please make sure to re-sync your data (or to import the priorly created backups of your local calendars).