Can't upload files in a document, but to the NextCloud

i have the problem, that if I try to upload, for example, a picture to a document opened in Collabora Online I get the error showing up in a pop-up Uploading file to server failed with status: 400. I can upload files directly to the NextCloud server over the app, webdav and the webpage and then add this to a document.
I am running the built-in Code Server for Collabora and have my NextCloud running with a Apache web server with the NC files in the root and behind a Nginx reverse proxy. Everything is running over https, the connection to the reverse proxy and also the connection from the reverse proxy to my web server for the NC.
I canโ€™t find any related errors in the logs of the nc and the browser just says POST https://[MYDOMAIN.EXAMPLE]/apps/richdocumentscode/proxy.php?req=/cool/https%[i guess unimportend stuff]&compat=/ws 400.

I hope anybody has an idea how this could be fixed.

Thanks for reading.

not exactly the same but my NC25 running with separated Collabora as Docker successfully adds images into writer doc through both ways copy&paste and Insert > local image

okay, then i will give it a try and setup an external server for that.