Azure Storage not used

Hi.

We are using .NET Core and have configured the Azure storage provider. We have upgraded our Xperience instance from hotfix 38 to 44 a few days ago.

Now we noticed that all images from the media library have vanished.

Uploading a new image results in a new file on the server in the folder C:\home\site\wwwroot\MediaLibraries\medialib.

Requesting the file by its getmedia / permanent URL from the admin instance works, but requesting it on the live app fails with a 404 since the live app tries to fetch the image from Azure storage.

Our settings:

Storage
 --------------------------------------------------

 Media libraries folder: ~/MediaLibraries/ (inherited)

 Use site-specific subfolders for custom media libraries folder: False (inherited)

 Media file hidden folder: __thumbnails (inherited)

 Media file thumbnail suffix: _thumbnail (inherited)

We have the following code in CMSModules/Azure/AzureStorageInitializer.cs, both in the live app and the admin app:

using CMS;
using CMS.Base;
using CMS.DataEngine;
using CMS.IO;

// Registers the custom module into the system
[assembly: RegisterModule(typeof(AzureStorageInitializer))]
public class AzureStorageInitializer : Module
{
    // Module class constructor, the system registers the module under the name "CustomInit"
    public AzureStorageInitializer()
        : base("CustomInit")
    {
    }

    // Contains initialization code that is executed when the application starts
    protected override void OnInit()
    {
        base.OnInit();

        // Creates a new StorageProvider instance for Azure
        var mediaProvider = StorageProvider.CreateAzureStorageProvider();

        // Specifies the target container
        mediaProvider.CustomRootPath = "mediadev";

        // Makes the container publicly accessible
        mediaProvider.PublicExternalFolderObject = true;

        // Maps the local media library directory to the storage provider
        StorageHelper.MapStoragePath("~/MediaLibraries/", mediaProvider);
    }
}

Also, CMSAzureAccountName as well as CMSAzureSharedKey are set up correctly, AFAICS.

There are no errors / exceptions visible in application insights on Azure.

What could be the reason for this behavior? How can I further debug this?

Thanks in advance!