[Dovecot] Configuration of dovecot 2.0.19 to authenticate users via LDAP

I am trying to get dovecot 2.0.19 authenticate users via LDAP (OpenLDAP 2.4.28) and using Wireshark to debug the process. It looks like the basic configuration of dovecot is fine, but it doesn't pull it off to send the right request to the LDAP server. First of all, this is how my LDAP-entries look:

# ht
dn: dc=ht
objectClass: top
objectClass: dcObject
objectClass: organization
o: ip
dc: ht

# admin, ht
dn: cn=admin,dc=ht
objectClass: simpleSecurityObject
objectClass: organizationalRole
cn: admin
description: LDAP administrator
userPassword:: e1NTSEF9cFY1b0ZZVUhack1aRTVvaUg1T3c2cytVWHV4aUNvaHI=

# people, ht
dn: ou=people,dc=ht
objectClass: organizationalUnit
objectClass: top
ou: people

# groups, ht
dn: ou=groups,dc=ht
objectClass: organizationalUnit
objectClass: top
ou: groups

# ipusers, groups, ht
dn: cn=ipusers,ou=groups,dc=ht
gidNumber: 500
cn: ipusers
objectClass: posixGroup
objectClass: top

# Max Mustermann, people, ht
dn: cn=Max Mustermann,ou=people,dc=ht
cn: Max Mustermann
givenName: Max
gidNumber: 500
homeDirectory: /home/users/mmustormann
sn: Mustermann
objectClass: inetOrgPerson
objectClass: posixAccount
objectClass: top
uidNumber: 1000
uid: mmustermann
userPassword:: e01ENX1ETUYxdWNEeHRxZ3h3NW5pYVhjbVlRPT0=
loginShell: /bin/bash
mail: mustorm@test.com

Now, I use the following configuration for dovecot (/etc/dovecot/dovecot-ldap.conf.ext)

hosts = 10.1.2.1
dn = cn=admin,dc=ht
dnpass = a
auth_bind = yes
auth_bind_userdn = uid=%u,ou=people,dc=ht
ldap_version = 3
scope = subtree
base = ou=people,dc=ht
user_attrs = homeDirectory=home,uidNumber=uid,gidNumber=gid
user_filter = (&(objectClass=posixAccount)(uid=%u))
pass_attrs = uid=user,userPassword=password
pass_filter = (&(objectClass=posixAccount)(uid=%u))

This is what I see in Wireshark: http://i.stack.imgur.com/ICzDe.png

Dovecot cannot authenticate itself for some reason...

If i change the configuration as follows:

auth_bind = no
#auth_bind_userdn = uid=%u,ou=people,dc=ht

Then I get following picture: http://i.stack.imgur.com/tb5vo.png

It doesn't pick up the setting base = ou=people,dc=ht. But what is more crucial is that even looking at the whole tree "<ROOT>" wholeSubTree it can't find the required entry.

I am really desperate and don't know how to make it work. Can somebody please give me a clue how to solve this problem?