Disable AutoSave by following the steps listed here: Turn automatic saving of draft messages on or off or change how often drafts are saved
If the issue occurs on a Shared Calendar, you can Turn on shared calendar improvements. After enabling shared calendar improvements, verify the calendar has been updated to Type (REST). If the shared calendar does not update, remove it and add it back using the following steps:
Right-click on the shared calendar and select Delete Calendar.
Close and restart Outlook.
From the Home ribbon, in the Calendar module, select Open Calendar > Open SharedCalendar.
Enter the name of the person who shared their calendar with you.
Select OK to close the dialog.