Hi,
Thank you for providing the configuration markup.
I was able to reproduce the problem with the Classic render mode. It happens because RadCheckbox is a new lightweight component that supports only the lightweight rendering and there is a mixture of render modes because of the RadButton CSS classes.
To solve the issue, please, switch to Lightweight to solve this problem:
Web.config
You can also set the RenderMode property inline inside the controls declarations.
Best regards,
Rumen
Progress Telerik