This seems to be a bug on the 7821..
http://www.cisco.com/c/en/us/support/docs/voice-unified-communications/unified-ip-phone-7900-series/100895-xml-parse-error.html
https://supportforums.cisco.com/discussion/12298921/cisco-7821-parse-error-4-when-accessing-corporate-directory
Also ensure you have the type command set on the phones
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