I doubt that the off-campus login would cause this.
Usually I've found that this complete is caused because the student used a practice login (guest login) -- but that wouldn't be true in your case if you find her name in the login file.
There is a transaction.log file which (depending on our global.conf entries) might be stored in .../courses/courseName/logs/transaction.log or webwork2/htdocs/logs/courseName_transaction.log which would tell you what actions the student submitted.
Sometimes students hit the preview button instead of the submit button (and hitting return instead of the submit button will trigger the preview button on most browsers).
Hope this helps.