I apologize for once again raising this issue. I've started Keycloak 2.5.4 server with default standalone-ha.xml config and "User Federation" worked! I guess, I need to remove all references to LDAP connections from production standalone-ha.xml config, start the server, and then re-add them. Issue resolved. Thank you for help. -- <https://www.youtube.com/watch?v=coVJlV1LJ84>