Encoding Error “zusammenführen”

Hello,

i have problems with the encoding (ä, ü, ö, …).

https://mattionline.de

Example: Widget title – Beiträge
Example: Popular Post – zusammenführen

The title of the post is displayed correctly within wordpress, but not within the widget.

How can i display it correctly with the popular posts plugin?

The page I need help with: [log in to see the link]