Unable to update customer's information in Plesk 10.x. The e-mail address field is highlighted and the following error is shown:
User with such e-mail address already exists in the system
The e-mail address firstname.lastname@example.org is associated with another user (Client1). Please specify another address
This is Plesk internal issue #PPPM-2652, which has been fixed in Plesk 10.4.4 microupdate #19.
Upgrade the Plesk installation to the latest version.