Modifying an existing user says The field 'Login' contains invalid characters!

With FusionDirectory 1.0.11 when I try to update a user having a dot in the login name (like marton.horvath) the update fails with error: 'The field 'Login' contains invalid characters!'. With version 1.0.8.9 this was working fine.

(from redmine: issue id 4761, created on 2016-05-02, closed on 2016-05-02)