@fbartels The thing is I can't check since the volume now works for me. But what I'm saying is that the issue was wider than just not being to write with the NC app, since data in the volume was not read / written into correctly anywhere (in NC app, in NC terminal and in Cloudron file browser) and that has nothing to do with the www-data user as far as I can tell.
And by the way you are right that the user in the terminal is root so it's true that being able to write with the terminal could still mean there is a restriction on the folder which would prevent the NC app to write. But as explained above, I don't believe this was the problem in my case.
