Unable to connect to LDAPv3 server - Connection failed to node '/LDAPv3/...' (2100)

This problem has bothered me for several years - because of this issue, I could not connect two of my mac machines to my ldap server. But I would like to understand this better and fix it.

Basically, I have an OpenLDAP server running on Ubuntu Linux (16.04) for all my lab computers. I have no issue using the LDAP authentication via Linux, but could not get it to work on my Macs.

I followed the instructions in

https://support.apple.com/guide/directory-utility/configure-ldap-directory-access-diru566b498a/mac

as well as several online tutorials, adding my sever using RFC2307 mapping and set the base search path as dc=server,dc=example,dc=edu, and in the Security setting, set Distinguished Name cn=admin,dc=server,dc=example,dc=edu, and typed in the correct password.

However, when I try to connect via Directory Utility/Directory Editor by choosing /LDAPv3/server.example.edu, it always gives me an error

Connection failed to node '/LDAPv3/...' (2100)

If I run the ldapsearch command to test my server, it worked perfectly fine in the terminal:

ldapsearch -x -D "cn=admin,dc=server,dc=example,dc=edu" -b "dc=server,dc=example,dc=edu" -H ldap://server.example.edu -W uid=testuser
Enter LDAP Password:
# extended LDIF
#
# LDAPv3
# base <ou=users,dc=server,dc=example,dc=edu> with scope subtree
# filter: uid=testuser
# requesting: ALL
#
...

I would like to know how to fix this - why ldapsearch is able to connect but the Directory Utility fails?