Signals Notebook: Unable to edit user's email address.

Sometimes when updating user's email in Signals Notebook, the following error message appears: "Editing user was unsuccessful", and the change is not saved.

REASON: One of the deleted users has the same email address.

WORKAROUND: Reactivate the deleted user, updated the email address to a different one there, and deleted that user again. After that, you should be able able to update the current user's email address.