Cannot open RadForm after updating Winforms

Hi, Peter,

I would like to ask you to check the references to all Telerik controls in your project and make sure that they are the same version considering the suffix as well (.20 or .40) since it is usually the most common reason for such a behavior after upgrading. Better yet, you can remove all references and add them anew by using the DLLs from your fresh installation: https://docs.telerik.com/devtools/winforms/installation-and-upgrades/updating-assemblies-in-a-project 
Delete the license.licx file. After that, you should rebuild your project, close Visual Studio and open it again to make sure that no references are kept in the memory by Visual Studio. Ensure that no old versions are available in GAC: https://docs.telerik.com/devtools/winforms/knowledge-base/examine-the-gac 
Off topic, the following KB article demonstrates the different approaches that you may follow when upgrading the Telerik UI for WinForms suite:
https://docs.telerik.com/devtools/winforms/knowledge-base/how-to-upgrade-a-project 

I hope this information helps. If you need any further assistance please don't hesitate to contact me. 

Regards,
Dess | Tech Support Engineer, Principal
Progress Telerik