Page’s password protection does not work (once more)

Hi
I know there a lots of topic about this issue and I tried many of the given solutions but I didn’t work for me.
I’d like to set a password protection for this page (incl subpages) https://www.diligans.biz/shop/
I’ve set a password but it is still available publicly, though it’s stated as “protected”.
The theme is ResponsiveBoat with the parent theme Zerif lite.
Can anyone help me?

Thanks
Michael