Thanks for updating the details. I was able to login and the issue seems to be because the mismatch in the URL of the dev site and it is still loading resources from a different URL.
The dev site link is:
https://www.pixelwerft-webdesign.de/physio and the resources are getting loaded from
Do you have an htaccess file that set the redirection of the URL? If so, kindly edit that file so that it will load the files from the dev site URL.
Let us know how it goes.