Please make sure that you don't have both RibbonBar and RibbonView assemblies in a single application because this is not allowed. Then in your xaml files, please remove the namespaces that references assembly particular version. The mentioned error indicates that you have namespace in xaml defined like so: "xmlns:controls= "telerik.windows.controls ... version="2011.3.920" .Remove all such namespaces and use the telerik scheme like so:
The last thing you have to do is to replace all occurrences of type telerikRibbonBar,telerikRibbonView, telerikX with "telerik".
Please let us know if this helped you.Regards,
the Telerik team
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