No sync in Spanish

Zotero does not sync with my Firefox 3.6.13 version in Spanish. It appears a message:
"You must enter your zotero.org username and pasword in the Zotero preferences to sync with the Zotero server."
When I accept, a window pop up:

Error de lectura XML: entidad no definida
Ubicación: chrome://zotero/content/preferences/preferences.xul
Número de línea 566, columna 2: <prefpane id="zotero-prefpane-cite"
--------^

What can I do?

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