I happen to have the same issue.
“samba-tool drs showrepl” shows the same issue. I have only one server.
ERROR(<class 'samba.drs_utils.drsException'>): DRS connection to ucs-8802.dom.yyyy.net failed - drsException: DRS connection to ucs-8802.dom.yyyy.net failed: (3221226047, 'No service is operating at the destination port of the transport on the remote system.')
univention-run-diagnostic-checks does not show any issues, besides the one above.
I appreciate any advice. I checked the article regarding the firewall ports
Thanks in advance.