Os dejos una solución para este error que encontramos en Vtiger, aunque pasa en cualquier aplicación o CRM que use código de PHP programado antes de la última versión. Parece ser que se han cambiado ciertas ordenes en PHP y ahora dan errores.
Para este error en concreto: PHP Deprecated: Function session_unregister() is deprecated  que encontramos en:
session_unregister(‘login_password’);
session_unregister(‘login_error’);
session_unregister(‘login_user_name’);
Se comenta con // y se pone:
 unset($_SESSION[‘login_password’]);
 unset($_SESSION[‘login_error’]);
unset($_SESSION[‘login_user_name’]);
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