It seems that the macOS app is no able to connect to an LDAP server on another port than 389.
In the LDAP-settings, there is no specific field to enter the LDAP-port; I tried to add the port to the server-url, like ldap://my-domain:1389 but this is ignored (I see failed attemps to connect to my primary LDAP-server at port 389).
My version: macOS Desktop 5.3.1 - Qt5.15.12 (Core 5.4.46-2-gbc9e72929)
When using the windows app, LDAP connection to port 1389 works flawless, so I assume that this is a specific macOS version problem...