Steps to reproduce:

1. Install R1 2021 SP1 (2021.1.204)
2. Create a Telerik WinForms project and drag a RadTostNotificationManager from the Toolbox and drop it onto the form. The following two references are required for using RadToastNotification:

   

   

   The important information here is the version of the Microsoft.Toolkit.Uwp.Notifications: 6.1.0.0

   

3. Run the Upgrade Wizard and upgrade to R2 2021 (2021.2.511):

After the upgrade process is successfully completed, you can see that the Toolbox version is successfully updated as well as the Telerik references in the project:

However, the version of the Microsoft.Toolkit.Uwp.Notifications still remains 6.1.0.0 even though in the installation folder we distribute the updated version 7.0.0.0:

Then, if you try to build the project, you will see the following error message:

Workaround: Remove the existing Microsoft.Toolkit.Uwp.Notifications assembly and add it anew using the references from the installation folder: C:\Program Files (x86)\Progress\Telerik UI for WinForms R2 2021\Bin40 It is expected to be the correct version 7.0.0.0.