Error 257 when opening file by tapping it in files app. Acessing the same file via the file browser works (Swift 5 - Xcode12)

Question

When I tap a file my app supports in the iOS file manger, my apps delegate method

private func open(url: URL, options: [UIApplication.OpenURLOptionsKey : Any]?)->Bool

gets called, where I read the file

do {
        try fileData = Data(contentsOf: url)
}
catch { 
        print(error.localizedDescription)
}

which gives an error 257 (not authorised). When accessing the same file using the document picker :

func documentPicker(_ controller: UIDocumentPickerViewController, didPickDocumentAt URL: URL) {
 if controller.documentPickerMode == UIDocumentPickerMode.import {
    var fileData : Data
    do {
         try fileData = Data(contentsOf: URL)
    }
    catch {
          print(error.localizedDescription)
    }
}

everything works fine. I get a data object which I can use. One thing I noticed is that the AppDelegate method gets an URL in the form of

/private/var/mobile/Library/Mobile Documents/com~apple~CloudDocs/Downloads/filename

while the documentPicker callback gets

/private/var/mobile/Containers/Data/Application/7A30D4D7-BD7D-4051-B3DE-143E18F48B23/tmp/myAppIndentifier-Inbox/filename

Adding - as suggested here -

let result  = url.startAccessingSecurityScopedResource()

did not make a difference. My "Supports opening documents in place" plist key is set to TRUE. Anyone any idea?