Windows Phone 7.5 camera app is crashing under WP8.1

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C#

Windows-Phone-7

Hello everybody,

I made a really simple camera app (it shoots a series of pictures in user-defined intervals), which was compiled for platform WP 7.1 (I think OS version 7.5 and 7.8) which works fine on my old ZTE Tania (it's running 7.8). I have uploaded it to the Store, it passed certification, but I see crashes under Windows Phone 8.1.

I see the following in the Dashboard:
Symbol Name:

Microsoft_Devices_Camera_ni!Microsoft.Devices.PhotoCamera.CheckInitAndCaptureComplete

Failure Name:

CLR_EXCEPTION_System.InvalidOperationException_80131509_Microsoft.Devices.Camera.dll!Microsoft.Devices.PhotoCamera.CheckInitAndCaptureComplete

I see, that somebody is using it under Windows 10 (Mobile) and there're no reported crashes for that system.

Is it a problem in code or some user-device specific issue?
This is my camera init code:

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
        {
            base.OnNavigatedTo(e);

            sw = new Stopwatch();

            timer = new DispatcherTimer();
            timer.Tick += new EventHandler(timer_Tick);

            cam = new Microsoft.Devices.PhotoCamera(CameraType.Primary);
            currentPhotoPath = string.Empty;
            lib = new MediaLibrary();


            cam.CaptureCompleted += new EventHandler<CameraOperationCompletedEventArgs>(cam_CaptureCompleted);
            cam.CaptureImageAvailable += new EventHandler<ContentReadyEventArgs>(cam_CaptureImageAvailable);


            cam.Initialized += new EventHandler<CameraOperationCompletedEventArgs>(cam_Initialized);

            viewFinderBrush.SetSource(cam);

            CameraButtons.ShutterKeyHalfPressed += new EventHandler(CameraButtons_ShutterKeyHalfPressed);
            CameraButtons.ShutterKeyPressed += new EventHandler(CameraButtons_ShutterKeyPressed);
        }

and handler for camera initialized event:

void cam_Initialized(object sender, CameraOperationCompletedEventArgs e)
        {
            Dispatcher.BeginInvoke(() =>
            {
                //get camera's supported resolutions
                availableSizes = new List<Size>(cam.AvailableResolutions);

                //update UI to show camera's supported resolutions
                resolutionPicker.ItemsSource = availableSizes;

                IsolatedStorageSettings iss = IsolatedStorageSettings.ApplicationSettings;

                //check if flash settings were saved and read them, or set to auto (UI index 0)
                flashPicker.SelectedIndex = iss.Contains(FLASH_SETTING) ? (int)iss[FLASH_SETTING] : 0;

                //read save resolution or set to the highest one
                resolutionPicker.SelectedIndex = iss.Contains(RESOLUTION_SETTING) ? (int)iss[RESOLUTION_SETTING] : availableSizes.Count - 1;
            });
        }

I have Visual Studio 2010 Express for Windows Phone installed and at this moment I can't install 8.1 tools to VS2013, but I don't know if repacking would help and I don't own 8.1 device to test it :) .

Thanks in advance!