As you know the content filters of RadEditor, parse and validate the content using regular expressions and DOM methods. If the content is non well formed or contains unsupported tags, attributes, etc this could break the regular expressions and the filter will pop up an error message.
You can validate the problematic content with the W3C validation tool of RadEditor or the online one and check for XHTML errors. You can also start to remove parts of the content until you find the problematic one in it and remove it.
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