Hello Sander, sorry for the late response.

That line is this:  $partDay = $excelTimestamp - $days;

So, I suppose there's something in the time stamp in the Excel file you used to generate this table that's causing this. Can you send me that file please?

This is a public ticket, so make sure to enable the PRIVATE response. Also, you won't be able to upload an XLS or an XLSX file, so make sure to ZIP it before attaching it to the ticket. Thank you.

Thank you Sander.

Strangely enough, I can use the file to generate the table without issues:

So, this could be related to your system setup. Can you please go to wpDataTables/System Info, and see if there are any warnings?

Reach out to your hosting provider, and ask them if you have ZipArchive installed, updated, and enabled? If not - have them install it, and restart the server after that.

If it still doesn't work, please provide me a temporary WP-admin (administrator) user for your website where this happens, 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 parties. 

You can write credentials here because this ticket is private and nobody can see them except us.