So I was having the same issue here. I deactivated all of my plugins and the issue vanished. After reactivating them one-by-one I was still able to save changes to my reoccurring event.
The strangest thing was happening though and I will create another ticket for this -> When I went back to my reoccurring event – beginning today (October 19) it changed the start date to December 29th EVERY SINGLE TIME I updated the event. This is just on the admin side.