Possible reason might be that there are elements that can not be translated from RSS to HTML.
For example if you copy and paste a word document you will received the same error, because the RSS can not translate the special symbols in the document. But the error you are getting is general error so can you please tell me is there a problem in the backend of Siteifnity or it is observing only in the log?
Please send us additional information about what is the pattern of reproducing the problem or what steps lead to it.
the Telerik team
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