ArgumentException with clarification similar to  "'\u001f', hexadecimal value 0x1F, is an invalid character." is thrown when trying to export document containing characters which are not supported in XML document - such as some control characters like 0x00, 0x1F, 0x1B, etc. Such characters are described in the XML specification here: https://www.w3.org/TR/xml/#charsets.
Although the escaped strings are not supported (see related 181318), the library could prevent the exception and export the document successfully by skipping such characters.
Workaround: remove such characters before the export. Check the following StackOverflow answer for some ideas on code for replacing the characters: http://stackoverflow.com/a/14323524/259206