On Sat, Oct 05, 2019 at 08:47:14AM +0200, Leo Führinger wrote:
>ldapdelete: /usr/local/lib/liblber-2.4.so.2: no version information available (required by ldapdelete)

This sounds like you are executing the Ubuntu version of ldapdelete
(/usr/bin/ldapdelete from package ldap-utils) but it is loading your
self-compiled libraries from /usr/local/lib.

The probable explanation is that your personal PATH includes
/usr/local/bin but the PATH used by the cron job does not.

You can solve that by modifying PATH in the cron job, but if you intend
to use self-compiled OpenLDAP, I recommend uninstalling the Ubuntu
OpenLDAP packages (ldap-utils and slapd) and using only your own.

As far as running newer OpenLDAP versions on Ubuntu, you may also
consider packaged OpenLDAP distributions offered by various community
members:

https://symas.com/symasopenldap/
https://ltb-project.org/
https://launchpad.net/~rtandy/+archive/ubuntu/openldap-backports (my own
unofficial PPA, updates the Ubuntu LTS packages to versions from later
releases)