[Samba] Replication issue

[Dave B]

Greetings,

I inherited a samba based domain at my work with two domain controllers
running Zentyal 3.4.8 with samba version 4.1.6-Zentyal. I don't know if
it's modified by the Zentyal team but they don't support this version
anymore, that's why I'm writing to this list.
The previos sysadmin told me that replication stopped working a while back,
but only in one direction. PDC gets replicated to SDC successfully but SDC
does not get replicated to PDC.

If I run the samba-tool drs replicate PDC SDC DC=mydomain,DC=lan
--full-sync command I get the following error:
ERROR(<class 'samba.drs_utils.drsException'>): DsReplicaSync failed -
drsException: DsReplicaSync failed (58, 'WERR_BAD_NET_RESP')

Running samba-tool drs showrepl command on SDC returnt no error, running it
on PDC returns this:
DC=mydomain,DC=lan
        Default-First-Site-Name\SDC via RPC
                DSA object GUID: 0a989f75-b8b8-4ae4-a6d3-b1a66fa1f895
                Last attempt @ Mon Dec  7 11:58:50 2015 CET failed, result
58 (WERR_BAD_NET_RESP)
                6922 consecutive failure(s).
                Last success @ Mon Dec  7 11:58:47 2015 CET

I've found similar problems, but no solution so far, so any help would be
appreciated!

Thanks in advance,
David