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The site that this log message appears on is configured for private files only. This message is generated every time the wysiwyg editor (ckeditor in this case) is displayed.
The message is generated by the unsuccessful call to file_unmanaged_save_data() on line 347 of wysiwyg.module as it is trying to write a file to public://js/wysiwyg/ which is not going to be possible in this case.