Apparently the install of the Telerik.Web.UI.dll assembly either did not occur or failed to make a necessary change to the site's web.config file during the upgrade. Please check the <runtime> node of web.config which should look like the following with the bindingRedirect for the Telerik.Web.UI.dll highlighted in yellow. If the newVersion is not correctly set to "2010.2.713.35" please change and re-save web.config:
<runtime>
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<probing privatePath="bin;bin\HttpModules;bin\Providers;bin\Modules;bin\Support;" />
<dependentAssembly>
<assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35" />
<bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0" />
</dependentAssembly>
<dependentAssembly>
<assemblyIdentity name="System.Web.Extensions.Design" publicKeyToken="31bf3856ad364e35" />
<bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0" />
</dependentAssembly>
<dependentAssembly>
<assemblyIdentity name="Telerik.Web.UI" publicKeyToken="121fae78165ba3d4" />
<!--Upgraded by DotNetNuke.Telerik.Web version 5.5.0 - Date: 8/23/2010 2:10:53 PM-->
<!--<bindingRedirect oldVersion="2008.0.0.0-2020.0.0.0" newVersion="2010.02.0713.35" xmlns="urn:schemas-microsoft-com:asm.v1" />-->
<bindingRedirect oldVersion="2008.0.0.0-2020.0.0.0" newVersion="2010.02.0713.35" />
</dependentAssembly>
</assemblyBinding>
</runtime>
Even with this change, you may have other issues due to the second error during the upgrade so check out all functions of your site thoroughly before concluding you have a successful upgrade.