Wow, this works!
Extra info:
at first, it did not work in a child theme. When i tried it in the main theme, it turned out it created a conflict with another plugin: easy watermark. after disabling this plugin, it worked. After that it worked as well in the child theme.
Thank you very much for your swift help. I can now continu to test the plugin.
Dear Fahim,
Thank you for your swift answer!
Regards, Ariën
Hi Tung Pham,
Thank you for your advice. The Force Update option was on NO all the time. Could it be an issue with the fact that the site was updated to WordPress 5.2.2?