downloading of series does not work anymore

www.kaleo.nl

I think I see why this happens. I can fix it tomorrow.

no hurry!

Can you check if the version 6.1.5 fixes it?

I think not.
Could there be something wrong in the php settings of our site?

No, the error was related to a database column size. Namely the "zip_size" of the table #__sermon_series. My guess was that your file size exceeds the value that could be stored in that field
The update did make that field much bigger. And in fact, the error doesn't appear anymore now.

Actually, it would work now. But the template doesn't show the modal for some reason. One thing is that the modal container has an "opacity" of 0, making it transparent.

From what I see, you're using the T3 Blank template which was written for Joomla 3. You should look to update that for a Joomla 4 compatible template. Then I think the modals should work as well.
You could also try with the default template (cassiopeia) to see if the download works with that one.