Hi Christian,
Welcome to the community.
Recently adware/ malware have been creating problem for Safari users.
Malwarebytes for Mac or Etreheck will help removing these adware malware.
Ref:
Bing https://discussions.apple.com/thread/250221302?page=1
Search engine changed to Yahoo? https://discussions.apple.com/thread/250149844
Safari wont quit?
https://discussions.apple.com/thread/7849754?answerId=31343998022#31343998022&ac_cid=sa123456#31343998
Cant force quit Safari? https://discussions.apple.com/thread/250108293
Developer of Malwarebytes for Mac, Thomas Reed and developer of Etrecheck ,John Daniels
are active members of this community.
These tools were developed for helping Apple Support Community members first
before they went mainstream.
Malwarebytes for Mac
Install guide: https://support.malwarebytes.com/docs/DOC-1817
Uninstall Guide: https://support.malwarebytes.com/docs/DOC-1190
EtreCheck: https://etrecheck.com/maspro
Etrecheck is a diagnostic tool
" Download EtreCheck: https://etrecheck.com/maspro
Click, “Free Download” button and then “Click to download” button,
open Downloads folder, click on it to open, and then select ”Open”.
Click on the bouncing EtreCheck icon in the Dock.
“Choose a problem” from the popup menu box, and then “Start EtreCheck” in the dialog.
If the report want to be posted here:
Click “Share Report” button in the toolbar and select “Copy report”.
Click the “Page” button in the toolbar at the bottom of this page and paste it when you reply."
Reading the report will help you help other users.
Test it yourself first.
Test it yourself
Best.