There's this strange behaviour in datepicker's date format choices;
Y-m-d is actually working, but d-m-Y as a date format is not.
I encountered the same problem. Both multisites on the same serve, the checks before export hit all the green lights but failed. Then I tried to import from the other end which gave the above mentioned error.
Any ideas would be appreciated!
I deactivated every plugin except forminator, and it did not work. After that test run I also deactivated the X theme and installed and activated Twenty Nineteen and still it did not work.