Where did you see this as an example? It is wrong in two ways
$pageroot must not have a trailing slash
$adminpages is no longer used.
If you change $pageroot after installing phplist then you will need to review and probably change a number of URLs on the Settings page, such as the subscription url.
I found this in an old forum I guess.
The trailing slash was just for testing. In fact I didn’t see the URL mapping in the Settings page.
Thank you for pointing me in the good direction !