hi @braunjulia welcome to the forum 
I have not many experience with built-in CODE but I think there is an issue with your reverse proxy and for this reason richdocuments looks for the wrong hostname.
Take a look here:
https://docs.nextcloud.com/server/stable/admin_manual/office/troubleshooting.html
and here:
Nextcloud Collabora integration
you will find lot of good troubleshooting hints