It looks like the firewall of the UCS 4.3 Samba/AD DCs is blocking TCP ports dynamically allocated by Samba 4.7. In our lab we found that the following adjustment fixed the issue:
ucr set \
security/packetfilter/package/univention-samba4/tcp/49152:65535/all/en="Dynamic RPC Ports (Samba)"
ucr unset \
service univention-firewall restart
Please note that this needs to be adjusted on all UCS 4.3 Samba/AD DCs.
We will also prepare an errata update to address this.