my blog listing page currently shows the featured images attached to my blog posts. (https://blog.servicetrace.de/) I would like to change that so that another pictures is shown in the blog listing - which would be a custome module, the "banner image":
You can retrieve the Banner_Servicetrace_November2019 module's banner_image.src value with this method. (The fact that the blog post and listing templates differ does not matter.)
For example:
{% for content in contents %}
{% if not simple_list_page %}
{% if content.widgets.module_157598967496270.body.banner_image.src %}
<img src="{{content.widgets.module_157598967496270.body.banner_image.src}}">
{% endif %}
{% else %}
<h2 class="post-listing-simple"><a href="{{content.absolute_url}}">{{ content.name }}</a></h2>
{% endif %}
{% endfor %}
In the example above, module_157598967496270 is your module's ID, so you should be able to plug it in.