It looks like an encoding problem. Are you able to reproduce the problem in the following live demo: Built-in Content Filters
If you are able to reproduce the problem in the live demo: Record your steps with Jing
and provide sample content for problem reproduction.
If you are unable to reproduce the issue in the live demo, please contact DNN and ask them for help. They have their own implementation of the DNN provider for RadEditor which Telerik does not support.
the Telerik team
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