Hello Tijmen

This can happen if your headers are too long, and the length of the headers goes above the defined limit in the database.

If this is the case, you'll need to change the origin header of the columns in the database as you won't be able to change those in wpDataTables.

Hi again Tijmen

Those are headers that are saved as headers in your database table.

You can open the table in your database management tool (phpMyAdmin for example), and check the header names in there. If they are long - shorten them in the STRUCTURE tab, under "Name":

Hello Tijmen

You did everything correctly, that should've done it, but I suppose the issue is coming from elsewhere.

Please provide me a temporary WP-admin login for your site where this happens, and database access credentials, so we could log in and take a look ‘from the inside’ as that’s the most efficient way to see and resolve the issue. 

We do not interfere with any data or anything else except for the plugin (in case that’s a production version of the site), and of course we do not provide login data to third party. 

You can write credentials here just check Private Reply so nobody can see them except us.

Also, could you duplicate the table again, so our developer can go through the steps on his own?