Line 76 in com_users/controllers/reset.php..
$this->setRedirect(JRoute::_($route, false), $message);
In the case of reset success, it tries to use variable $message without ever defining a message, causing a PHP error to be logged.
This is actually the same as tracker bug
[#25423] $message not initialized in ~/components/com_users/controllers/reset.php
from a year or so ago. #25423 was closed without a fix as "not in Joomla core" because someone mentioned unrelated component com_accounts in the incident. But it is actually a core com_users issue.
I see this problem on 1.7.3, and I just verified that it is still a problem in latest 2.5 code.