Cannot read property 'oSort' of undefined

I have added a custom sort functionality to my datatable.

and assigned the colum like this
table = jQuery('#table').DataTable({ columnDefs: [{ "type": "weekday-sort", "targets": 2 }],...

It worked smoothly many month, but now suddenly if receive the error message:

I am not aware that I changed any code or so, but obviously something changed.
I tried to use different datatables.min.js files and stuff, but the error keeps persisting.

When I remove the custom sort function the datatable initializes correctly. So something has to have changed, but I have no clue what.

Please help.