Sorry for the late response. My life has been a mess recently and I didn't have time.
I was able to track down the problem using the options you provided — it showed me the error ": config_add_internal: DN="cn=config" already exists". I found the solution on the mailing list: https://www.openldap.org/lists/openldap-software/200711/msg00068.html.
Thank you for your help.