It seems that the problem concerns the changes that are made in the web.config during the upgrade process.
Is it possible to send us the project (or an example one) that reproduces the problem, so that we can examine it? This will be very helpful and we will do our best to fix the issue for one of our upcoming releases.
Additionally, could you describe the exact steps that you went through to get this error? What was your old version? What is the version you upgraded to?
Looking forward to your reply.
the Telerik team
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