Below is a quote from my response in your support ticket on the same question:
"Such error can occur in the VS Report Designer if the projects having references to Telerik Reporting are not upgraded. Please test to run the Upgrade Wizard and restart Visual Studio - Upgrading Telerik Reporting Projects.
Please let us continue the discussion in one of both threads in order to keep a better track on the exchanged information and the progress of the issue.
Thank you for your understanding.
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