Site not reachable (backend working)

Hi guys!
I’ve been trying already for a few hours but I can’t fix the issue. I even think the more I touch, the more I destroy =/

My website:
https://www.almased-alternativen.de/
https://almased-alternativen.de/

The issue started when I tried to redirect from the non www version to the www version.
I changed the WordPress Address (URL) in Settings >> General to the www version. I also changed my .htaccess and that worked. The website was redirecting from the non www to the www version.
The problem was that the www version wasn’t existing so I got a 404. Strange.
That issue made it impossible to login to the backend, so I modified functions.php from my active theme. That worked. I changed back functions.php.
I changed back the WordPress Address (URL) in Settings >> General the same way it was before but I still can’t see my website. I get in both versions a 500 Server Error.

Any idea how to solve this? Thanks a lot!!!