Some files are not writable

I am currently working on a project where I need to install Kirby on a self-hosted server for the first time. On commercial web spaces, drag and drop works without any issues. However, I spent a few days working with the server admin to get Kirby up and running. Everything seems to be functioning now, except for one issue:

I am unable to edit pages through the backend. I receive the error message: ‘The file is not writable.’ Strangely, I can create and save new subpages and edit them, but this functionality doesn’t work for pages that were copied over from the test server. I’ve already checked if the .txt files have any unusual settings and replaced them with fresh ones, but even these cannot be edited via the panel.

What else do we need to configure correctly (presumably on the server side)?

Hello,

Sounds potentially like a permission issue at the server side.
Your files should be be set to 644, directories should be 755

Might be worth checking this just to be sure.