Thank you for your fast response.
To complete my message:
The Server is running as CentOS release 5.2 with Postgresql 8.2.9. from RPM, aolserver 4.5 from source and openacs 5.4.
I'm using openldap-devel-2.3.43. For ns_ldap I tried the version from Malte Sussdorff (http://www.sussdorff.de/ressources/nsldap.tgz) and the newest (from 2002) from sourceforge (v0r8)