Steps to reproduce:

1. Log in as an Administrator
2. Site Administration > Users > Browse list of users > Choose a user > profile
3. Edit the password (hit enter as the page suggests) and hit Save

This results in an error 500

(This has been noticed if 'Disable Notifications' is checked for that user, possibly even otherwise, not tested for too many other cases, tested only on current stable 4.5.1.)