Bug was encountered during implementation test for CORE-5904.
After discuss with Alex it was decided to create separate ticket for this.

After launch cmd.exe and change there code page from OEM to ANSI (1251 in my case) i run following script:
===
    set echo on;
    set names win1251;
    shell del C:\MIX\firebird\QA\fbt-repo\tmp\c5904-w1251.fdb 2>nul;
    create database 'localhost:C:\MIX\firebird\QA\fbt-repo\tmp\c5904-w1251.fdb' user sysdba password 'masterkey';
    create or alter mapping foo using plugin win_sspi from user 'org\фуу' to user "org\foo";
    rollback;
    drop database;
    exit;
===

-- and get:
===
    Statement failed, SQLSTATE = 22000
    unsuccessful metadata update
    -CREATE OR ALTER MAPPING FOO failed
    -Malformed string
==
Checked on:
  WI-V3.0.4.33053
  WI-T4.0.0.1224