Solution: switch TLS on with ldaps
In my case, I had to switch TLS and ldaps on via following parameters:
# Set to true if LDAP server should use an encrypted TLS connection (either with STARTTLS or LDAPS)
use_ssl = true
# If set to true, use LDAP with STARTTLS instead of LDAPS
start_tls = false
To find this out, ldapsearch was helpful (in my case the url had to be set with ldaps, so it beca,me clear)
https://docs.ldap.com/ldap-sdk/docs/tool-usages/ldapsearch.html