In a domain with two DCs samba-tool drs showrepl on DC2 showed WERR_SEM_TIMEOUT for the inbound replication from DC1 and replication stalled.
As it turned out, the DC1 had three network interfaces configured for three separate /24 networks, and Samba was binding to all of them. As a result the dnsupdate process on DC1 wrote all 3 IPs into DNS.
DC2 in contrast had a single network interface configured for only one of the subnets but it persistently attempted to connect to DC1 on one of the IPs that were not reachable.
Probably Samba should be more graceful in this situation and attempt to use all registered IPs and prefer those that are reachable.
*** Bug 10175 has been marked as a duplicate of this bug. ***
(In reply to Andrew Bartlett from comment #1)
This was in error, Sorry.