What’s your date format?
I have a similar issue Events not being created / date misparsed
Was this an event before September 13?
When the month and day are swapped, dates Sept 1 - Sept 12 are valid dates (Jan 1 - December 1) but September 13 becomes invalid (there is no first day of the thirteenth month).
I bet you can create an event between October 1st and October 12th.