[Samba] unexplained Replication failures...?

[vincent at cojot.name]

Hi all,

I'm running in circles trying to debug replication failures on samba 
4.7.6:

dc00 : is a VM on KVM host (attached to a bridge on local LAN)
dc01 : is a similarly configured VM on another KVM host.

I've forcibly demoted and re-promoted dc01 but I still cannot get 
automatic replication to work:

root at dc00 ~]# samba-tool drs showrepl
Krynn\DC00
DSA Options: 0x00000001
DSA object GUID: 204cb904-754b-4457-af09-9347f8714006
DSA invocationId: b72fc409-bf9a-45e2-a623-0e668386536a

==== INBOUND NEIGHBORS ====

DC=ForestDnsZones,DC=ad,DC=lasthome,DC=solace,DC=krynn
         Krynn\DC01 via RPC
                 DSA object GUID: 9ac5b74a-383a-4336-9c5d-978b45bad9c9
                 Last attempt @ Thu May  3 18:50:52 2018 EDT failed, result 87 (WERR_INVALID_PARAMETER)
                 4 consecutive failure(s).
                 Last success @ NTTIME(0)

All of these show 'Last success @ NTTIME(0)'.

I can force replication manually just fine but automatic replication 
doesn't seem to work.

[root at dc00 ~]# samba-tool dbcheck
Checking 351 objects
Checked 351 objects (0 errors)
[root at dc00 ~]# samba-tool drs replicate DC01 DC00 dc=ad,dc=lasthome,dc=solace,dc=krynn --sync-forced --full-sync
Replicate from DC00 to DC01 was successful.

Any ideas?

Vincent