WPP Widget

There are a few issues there, some styling related due to your theme interfering and some WordPress related. Let’s fix the latter first:

#1 Replace the Text widget you’re using with a Custom HTML widget.

#2 Set Custom HTML widget’s title to “Meistgelesen”.

#3 Paste your [wpp] shortcode into the Custom HTML widget.

#4 Add this line of code to your site, either via your theme’s functions.php or by using a plugin such as Code Snippets for example:

add_filter( 'widget_text', 'do_shortcode');

It’ll allow the Custom HTML widget to parse shortcodes.

Once you have done all that please let me know and we’ll look into the styling part afterwards.

That’s PHP code.