server error 500 on trying to reset password in Moodle

A Moodle admin reported internal server error 500 on trying to reset the password for another user. And confirmed that the same error occurs even when resetting his own account's password from the profile field. But the password reset via the login page works.

So, perhaps there is some setting in Moodle which prevents anyone from changing passwords except via the password reset page
or
maybe there's a bug
or
there's some issue which prevents updating of user records.

I could reset the password using the commandline,
https://docs.moodle.org/405/en/Administration_via_command_line#Reset_user_password

which in our case is

sudo -u www-data /usr/bin/php admin/cli/reset_password.php

# this prompts for the username and then for the new password.