There shouldn`t be such problem occuring after the upgrade. Unknown server tags is an error producing when for example you use a custom control on a master page and the assembly for this control is not referenced. When upgrading Sitefinity all Sitefinity related assemblies are replaced with their newer versions and then the project is built with the latest assemblies in place.
If you have controls comming from external assemblies make sure those assemblies are still referenced in the bin folder of the project.
the Telerik team
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