I have found the issue.
Previously settings have been used for extra block creation, now it's customSettings
Changing everything accordingly fixes issue.
Unable to add a user named in Russian letters to the group
It just can't find such of user.
As MongoDB full text search is in beta, it has some limitations. This has been already addressed to the MongoDB devs: https://jira.mongodb.org/browse/SERVER-8423
After this user is renamed himself with English, it still impossible to find and add him to the group.