I think there may be an issue in the mapping between the Synology LDAP and Cloudron LDAP If you look at the first screenshot provided by imc67, we have the option to select which LDAP profile to use. If you use the standard profile, you immediately run into the issues that imc67 described. However if you change the profile to custom (see screenshot below), everything checks out fine except for the "Check server schema" which goes away if one enables CIFS plain text authentication. But then you still get stuck when the Synology tries to join the Cloudron directory.
I'm trying to find the logs on the synology to get more details.
Here are the synology docs for how to join a Synology NAS to an external LDAP directory: https://kb.synology.com/en-me/DSM/help/DSM/AdminCenter/file_directory_service_join?version=7