Authenticate against Ubuntu 16.04 active directory

[miboco]

Hi all,
I´m fighting with this one and can´t find any solution to authenticate users against a Samba AD server running on Ubuntu. Here is what I´ve got:
Alix Box with pfSense 2.4.3
Ubuntu 16.04 with Samba 4.3.11
The settings of the ubuntu server:
dns: ubu-ad.mydomain.loc
realm: mydomain.loc
Domain: mydomain
What´s working: joining domain with a Win 7 Pro box, logging in to a Win 7 Pro box with any user I´ve configured on the ubuntu box, full administration of the domain from a Win 7 Pro box with the MS RSAT tools.
Telnet connection to the ldap port
Ping to ubu-ad.mydomain.loc from the pfSense box
I´ve set up the LDAP configuration on the pfSense box as described in "The pfSense book" but can´t bring it to work. "Authentication" always fails and when I hit the "Select a container" button in the setup field I get an "Could not connect to the LDAP server. Please check the LDAP configuration." error message. For my configuration please have a look at the attached screenshot.
Any help is welcome!

Greetings,
Mike