For some reason the address to your document server is not correct/ valid. I had the onlyoffice app installed before, installed the community server afterwards and that wrote the correct address into that field automatically.
The address should be like:
https://nextcloud.your-domain.de/apps/documentserver_community/
Your entries under “Erweiterte Servereinstellungen” don’t seem right as well.
Try this but adept to your domain:
