I had the same issue, an XML error can mean your mailing address, postal code, SIN or some personal info is incorrect. I went back to the beginning and went through my personal info again. No mistakes in it but after going through  it I was able to file. Sometimes this error happens if you use auto fill. 

the question is under " 'your name' Canadian Citizenship" in the 'my info' section

This issue will be fixed shortly- for now you can  go to "My Info" click on (Name) Canadian Citizenship and make sure that you answer the questions " Do you authorize the CRA to share your name and email address with the Government of Nunavut to contact you about organ and tissue donation, and to maintain the organ and tissue Donor Registry?

This happened to those who transferred from last year.

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