“page not found” after adding group page

Hi,

I created a trial page in one of the groups.
The page should be visible but it does not appear in the menu of the group.
When I try to access the page here
https://www.scarsellini.it/wp/gruppi/sale-comuni/gpages/prova-pagina/
I get “page not found”.

The page is listed in “group pages”, it seems published there… but no success in reaching it.

What am I missing?

Thanks a lot.

Carlo