Redmine

I've successfully configured ldap connection from redmine to zentyal 4, when I click "test" it works (but column "users" is set to 0)
but when I try to login, it gives me error "username or password not valid"
in redmine log I found:

Failed login for 'DOMAIN\admin' from 192.168.10.30 at 2016-11-11 13:38:09 UTC
Rendered plugins/redmine_tweaks/app/views/account/_login_text.html.erb (0.3ms)
Rendered account/login.html.erb within layouts/base (4.1ms)
etc. etc.

where can I found more details to understand the problem?
thanks