Help me understand this stacktrace

Like the title says, I'm having issues understanding a stacktrace I receive from HockeyApp.

Xamarin_iOS_Foundation_NSUrlSession_CreateUploadTask_Foundation_NSUrlRequest_Foundation_NSUrl
objc_msgSend() selector name: isFileURL
NSUrlSession.g.cs, line 455

The code (I think) that triggers it is the following:

        var path = "...";
        File.Copy(toCopy, path);

        NSUrl uploadHandleUrl = NSUrl.FromString(sync.Url);
        NSMutableUrlRequest request = new NSMutableUrlRequest(uploadHandleUrl);
        request.HttpMethod = "POST";
        request["Content-Type"] = "multipart/form-data; boundary=\"" + BaseUploadService.Boundary + "\"";

        var uploadTask = this.syncSession.CreateUploadTask(request, NSUrl.FromFilename(path));

The stacktrace is added as attachment, as it was too large to post here.

Now this code is called in a Parallel.foreach as I have many of these that get started.

Does that error indicate that the file does not exist ?