Older versions of Moodle do not have an answernumbering value for their questions, but may include the answeringnumber XML entry when exporting as Moodle XML. A more recent Moodle that does use answernumbering will read the blank value and store it in question_multichoice.answernumbering without validation.
The visible result: In these imported multichoice questions, the text "ERR" will prefix all of the answers' text, as in the attached screenshot.
The fix is trivial, simply check to see if the answernumbering is defined and if not, set it to the default (abc). I've attached a patch for question/format/xml/format.php