Outlook error message copying calendar entries between calendars

I have a work calendar. I have a private calendar.  And I am the member of an association as well.  Sometimes I need to copy  calendar entries from my work or association calendar to my private calendar.  Up until a few days ago, all I had to do was select the appointment in the appropriate calendar, and drag it to the main one.  Outlook would create a copy in my main one, leaving the original appointment where it was.  Now, I get an error message saying that the copying of appointments is not supported.  How do I get the functionality back?

I have seen that other people are complaining about this