LDAP authentication not working

Hi,

I'm running postgresql 9.1 on Debian and am trying to set up LDAP
authentication using the following configuration in pg_hba.conf:

hostssl testdb all 143.50.203.0/24 ldap ldapserver="wegc24.uni-graz.at"
ldapport=636 ldapbinddn="cn=nss,dc=uni-graz,dc=at"
ldapbindpasswd="<thepasswd>" ldapbasedn="dc=uni-graz,dc=at"

Trying to access testdb via psql fails with the following error in the log:
'''could not perform initial LDAP bind for ldapbinddn
"cn=nss,dc=uni-graz,dc=at" on server "wegc24.uni-graz.at": error code -1'''

Unfortunately I did not find what error code -1 means.

Ldapsearch works fine:
> ldapsearch -W -H ldaps://wegc24.uni-graz.at:636/ -D
"CN=nss,DC=uni-graz,DC=at"

Interesting is also, that postgres seems to not even reach the ldap
server: If I change parameter ldapserver to a non-existing url it gives
the same error code -1.

Any help much appreciated!

Best,
Juergen