This is not a new issue, it has been rarely and randomly affecting upgrades for a long time. I have looked into this several times and never found a pattern of why some sites have problems and others do not. I have never been able to reproduce this on demand.


The telerik failure is usually caused by a failure to update the web.config file.


To fix this:

Verify the Telerik.Web.UI.dll version from the website bin directory.
Go to root directory of the website
Expand Bin folder
Right click Telerik.Web.UI.dll and select properties
Click the details Tab, Product Version should be 2010.03.1317.35

If the .dll is not the correct version, you can get the latest from:

\Install\Module\Telerik_05.06.02_Install.zip\bin\ in the upgrade.zip
or
\bin in the install.zip


In Web.config, look for




Ensure that the Web.config has the newVersion set to the same as the .dll in your bin directory. Be aware that there will likely be several commented out bindingRedirect entries, the only one that counts is the one that is NOT commented out.

Same problem in several upgrades to CTP 3. I had to do a binding redirect to get the correct reference to the current telerik dll, and it didnt work untill I rebuilt the project in Visual studio and cleared all cache.
Wrong! A reference in the core somewhere to the old dll, and it should not have any references anywhere to the telerik dll except in the web.config?????????
Telerik resources 6.0 always failed install on  upgrades from 5.6.2 to CTP 2 and/or CTP 3. My stock web config is looking for version 2010.03.1317.35 and not 2010.2.929.35. Even on a successful installation of the new 6.0 Telerik resources it does not update the web.config reference.