'Invalid argument' when trying to use a background URLSession for a Download Task

I am working on an application that requires to download a certain number of files to be able to work offline. Obviously, download tasks are preferred to be done with the app in the background. I implemented an URLSession with a background configuration following Apple's documentation available here : https://developer.apple.com/documentation/foundation/url_loading_system/downloading_files_in_the_background. I also followed a tutorial on raywenderlich: https://www.raywenderlich.com/3244963-urlsession-tutorial-getting-started.

Basically, what I've done looks like this (I've made my class a Singleton but I have the same problem either way):

public final class DownloadService: NSObject {
    static let shared = DownloadService()

    static let identifier = "downloadService"

    private var urlSession: URLSession!

    var backgroundCompletionHandler: (() -> Void)? // This is attributed in the handleEventsForBackgroundURLSession delegate method in the AppDelegate

    private override init() {
        super.init()

        let config = URLSessionConfiguration.background(withIdentifier: DownloadService.identifier)
        config.isDiscretionary = true
        urlSession = URLSession(configuration: config, delegate: self, delegateQueue: nil)
    }
}

extension DownloadService: URLSessionDelegate {
    // Delegate method called when the background session is finished.
    public func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
        DispatchQueue.main.async {
            guard let completionHandler = self.backgroundCompletionHandler else {
                    Logger.fault("No completion for bg session", category: .network)
                    return
            }
            Logger.log("Complete background session", category: .network)
            // This must be executed on the main thread
            // Executes things such as updating the app preview in recent apps view
            completionHandler()
        }
    }
}

extension DownloadService: URLSessionDownloadDelegate {
    // Delegate method called when a download task is finished
    public func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
        // Perform
        guard let sourceUrl = downloadTask.originalRequest?.url else {
            return
        }
        Logger.log("Received file: %@", sourceUrl.lastPathComponent, category:.network)
        // Check and save file
        saveFile(originalFileURL: sourceUrl, downloadedTo: location)
    }
}

And I start the download using:

/// Download file using a previously created URLSession.
/// - parameter filename: Name of the file.
/// - parameter baseURL: URL where the files are located.
/// - parameter size: Expected filesize in Bytes.
private func download(file filename: String, from baseURL: String, size: Int64) {
    guard let url = URL(string: baseURL)?.appendingPathComponent(filename) else { return }
    let task = urlSession.downloadTask(with: url)
    task.countOfBytesClientExpectsToSend = 0
    task.countOfBytesClientExpectsToReceive = size
    task.resume()
}

My problem is that everything works fine when the app is in foreground, but whenever I put the app in the background or lock the screen, I have an error saying:

Task <46648342-7D13-4D1F-96A1-FDAE4C1F8475>.<362> finished with error [22] Error Domain=NSPOSIXErrorDomain Code=22 "Invalid argument"

I have tried playing a bit with the URLSessionConfiguration, specifically the isDiscretionary parameter which is set to false by default, and it seems that setting it to true, as advised by Apple's documentation, even blocks the download from proceeding with the app in the foreground, resulting to the same error 'Invalid argument'. I wonder if this parameter has anything to do with my problem, or if there's something I've misunderstood? The exemple on raywenderlich provided above also works the same way, using isDiscretionary seems to make the download fail everytime.

I am using Xcode 11.3.1 with Swift 5 and targeting iOS13.

Let me know if any other information is needed and thank you for your help!