"User does not exist."
To complete email address update, you should verify the updated email address by clicking the verification link in the "Verification mail for Shifter" mail.
A "Verification mail for Shifter" mail will be sent after your request to change your email address. Make sure the email slips into Junk mail box.
If you haven't got the email "Verification mail for Shifter", feel free to contact the support desk.