Error #0264: you can't change the email, the address "XXX" is already used in another account
Impossible to change the user profile because the email address is already used by another account.
Check if you specified the address correctly and it is does not match with other users' addresses in the tool "User management". Use a quick search filter.
If you need to allow duplicates, disable control of email addresses matches in the server's settings. By default, this configuration is disabled.