Book navigation block inactive

I am sorry but I can not reproduce this behavior. It seems it is related to your installation/configuration and not to the theme itself.
Try to configure the block as "Show block on all pages" and see what happens.

The book navigation block is already configured to "Show block on all pages".

Today I made a new installation with an older version of drupal (8.9.20) / zero point theme (8.x-1.5) - same issue.
For testing purposes I have added another theme (Tara 9.2.0), and after activating the sidebars, assigning the blocks to the sidebar left (similar to sidebar first at zero point theme), everything went ok with the testing theme tara.

So I've added the tara theme (10.0.4) my drupal 10 installation and everthing went ok too with tara.

In both cases, no other things were activated or changed.

Strange.... is there something more I need to check ?

Solution is simple. Clear cached data at /admin/config/development/performance or "Flush all caches" in Admin Menu.

I've changed my configuration on my current website as described above , and everything went ok.
Now I've created a completely new site, made a clean installation and everything works like a charm.

Thank you, Florian.
Marked as fixed.