I resolved this issue.  The cause seems to have been that I had 2 instances of the  event module installed on the page - one was configured to display a month calendar, the other one was configured to display a list of upcoming events.  They both used the same event information so when I added an event it would show up in both the list as well as a calendar. 

After I removed the module that was set to display the calendar format, I could then easily add, edit, update, or delete any event without problem.

The 2 instances had worked previously, and I believe that kind of setup was even one of the configuration examples that I had found on the DNN site.  But the evidence shows that this is no longer supported, at least not the way I had been using it.

"Thank You" to everyone who helped me get to a work around for the issue!