PHP 8.1 - Fatal error: Uncaught Error: Undefined constant "USERNAME" in ***\ehandlers\mail.php on line 451

[Jimmi08]

Bug Description

Fatal error: Uncaught Error: Undefined constant "USERNAME" in ***\ehandlers\mail.php on line 451

Fatal error with debugging emails on (LOG and SEND option) after the user (main admin in this case) is logged in .

How to Reproduce

set notify about user logged in
set email debugging on

Thanks