iOS Files App/extension shows "Content unavailable" when you have too many items in a folder

Steps to reproduce

1. Extract the attached content, Files App Testdata.zip in your Nextcloud folder
2. Go to your iPad/iPhone, open Apple Files App
3. You might be able to open Nextcloud -> Files App Testdata -> 500 which contains 500 PNG image files,
4. But you will probably fail to open Nextcloud -> Files App Testdata -> 700,

Expected behaviour

You should be able to access any folder from your Nextcloud instance.

Actual behaviour

When a folder contains too many files, Files App is unable to display any content. In my test case I was never able to open a folder with 700 or more files, the folder with 600 images sometimes works (but often fails, too).

Note

I did the same test with Microsoft OneDrive and DropBox and had no problems. Seems like OneDrive had a similar problem before (https://www.reddit.com/r/onedrive/comments/hr08ld/ios_files_app_content_unavailable_if_more_than_50/g0b7xm4/) but they fixed it. So it should work in Nextcloud, too.

Environment data

iOS version: iOS 14.4.2 & 14.5

Nextcloud iOS app version: 3.4.2.0

Nextcloud version: 21.0.1

Start by reproducing the problem in the Apple Files App with the attached Files App Testdata.zip, comparing the 500-file and 700-file folders. Inspect the Nextcloud iOS app's Files App extension entry point and its handling of large folder listings; done means folders with 700 or more files can be opened and displayed reliably.