NOTE: This only affects the deletion of users where the email field is empty - e.g. in cases where the account was created via an import.

Line 109 of user/lib.php

if ($user->username !== core_text::strtolower($user->username)

The code above is comparing values AND types.
When I get the error with 'usernamelowercase' it isn't because the strings don't match, its because $user->username is a double and the value returned by core_text::strtolower($user->username) is a string.

There are many possible fixes for this

1) remove an equals sign from line 109 of user/lib.php

if ($user->username != core_text::strtolower($user->username)

2) force a string value on the username on line 109 of user/lib.php

if (strval($user->username) !== core_text::strtolower($user->username)

3) force a string value on the username on line 4249 of moodlelib.php

$updateuser->username = strval($delname);