radekzie

Problem solved. The fault was on my side – I added spaces after commas between the page IDs. Interestingly, I was convinced that your plugin initially worked for all the sites I defined. Maybe it wasn’t like that?
Thanks for the answers and sorry for your lost time.

Thank you for your reply.

Have you ever change the pages that you defined to “post page” on the reading section?

No.

Or somehow the pages have rearranged to posts by using different plugins?

I think no.
The pages that should be protected are:
https://sydor-zielinska.pl/o-mnie/
https://sydor-zielinska.pl/ukrainian/
https://sydor-zielinska.pl/english/

Plugin works only for the first page listed.