Ok I finally got it to work. I share my findings here. It was all about the route and a missing results page.
My structre is like this:
01,blog
–>blog.md
04.books
–>books.md
–>book1
---->book.md
…
in my simplesearch.yaml I have:
route: /search
search_content: rendered
template: mysearch_results
filters:
filter_combinator: or
my blog.md looks something like:
title: Blog
content:
items: '@self.children'
order:
by: date
dir: desc
limit: 5
pagination: true
pagination: true
simplesearch:
route: /search
filters:
- @self
filter_combinator: and
my books.md looks like this:
title: Books
content:
items: '@self.children'
order:
by: date
dir: desc
limit: 20
pagination: true
pagination: true
simplesearch:
route: /search
filters:
- @self
filter_combinator: and
in both, my blog.html.twig and books.html.twig I include the sidebar with:
{% include 'partials/sidebar.html.twig' with {'page':page} %}
!!Attention: In this two files I have to delete the change of the base_url, which was:
{% set base_url = page.url %}
This line caused the problem on the books.md. page, so the search result never was shown.
in the sidebar.html.twig the searchbox is included with this command lines:
{% set new_base_url = page.url == '/' ? '' : page.url %}
{% if config.plugins.simplesearch.enabled %}
<div class="sidebar-content">
<h4>SimpleSearch</h4>
{% include 'partials/simplesearch_searchbox.html.twig' %}
</div>
{% endif %}
and the simplesearch_searchbox.html.twig remains unchanged.
Hope it helps you too. Good luck.