The DFS Replication service is stopping communication with partner DC3 for replication group Domain System Volume due to an error. The service will retry the connection periodically.

Not a lot to go on but I'd check that the required ports are flowing between networks.
https://learn.microsoft.com/en-us/troubleshoot/windows-server/identity/config-firewall-for-ad-domains-and-trusts
https://www.microsoft.com/en-us/download/details.aspx?id=24009

--please don't forget to Accept as answer if the reply is helpful--

Hi,

Did the errors happen at the specific time?
Was there a backup processing when the error happened?
If the error was caused by the backup, you can ignore the error or schedule the replication to stop when the backup processing.
Following link for your reference:
https://social.technet.microsoft.com/Forums/en-US/af89b4e2-ad28-4978-8355-3973cc476dfe/dfs-replication-issue-in-secondary-ad?forum=winserverDS

Best Regards,

Just checking if there's any progress or updates?

--please don't forget to Accept as answer if the reply is helpful--

DSPatrick
Not a lot to go on but I'd check that the required ports are flowing between networks.
https://learn.microsoft.com/en-us/troubleshoot/windows-server/identity/config-firewall-for-ad-domains-and-trusts
https://www.microsoft.com/en-us/download/details.aspx?id=24009
Not sure what to do with your first link.
Client Port(s) Server Port Service
49152 -65535/UDP 123/UDP W32Time
49152 -65535/TCP 135/TCP RPC Endpoint Mapper
49152 -65535/TCP 464/TCP/UDP Kerberos password change
49152 -65535/TCP 49152-65535/TCP RPC for LSA, SAM, NetLogon ()
49152 -65535/TCP/UDP 389/TCP/UDP LDAP
49152 -65535/TCP 636/TCP LDAP SSL
49152 -65535/TCP 3268/TCP LDAP GC
49152 -65535/TCP 3269/TCP LDAP GC SSL
53, 49152 -65535/TCP/UDP 53/TCP/UDP DNS
49152 -65535/TCP 49152 -65535/TCP FRS RPC ()
49152 -65535/TCP/UDP 88/TCP/UDP Kerberos
49152 -65535/TCP/UDP 445/TCP SMB (**)
49152 -65535/TCP 49152-65535/TCP DFSR RPC (*)
I am guessing these ports have to be enabled. I open Windows Defender Firewall.
I look at inbound rules
123/UDP enabled
135/TCP enabled
464/TCP/UDP enabled
49152-65535/TCP can not find
389/TCP/UDP enabled
636/TCP enabled
3268/TCP enabled
3269/TCP enabled
53/TCP/UDP enabled
49152 -65535/TCP can not find
88/TCP/UDP enabled
445/TCP enabled
49152-65535/TCP can not find
Tried portqry -n DC3 -p both -e 49152-65535 but stopped that as it was taking too long for the results.
123 listening
135 not listening
464 listening
389 listening
636 TCP listening UDP not listening
3268 not listening
3269 TCP listening UDP not listening
53 listening
88 TCP listening UDP not listening
445 TCP listening UDP not listening

Did the errors happen at the specific time? When I run the program.
Was there a backup processing when the error happened? Not that I know of. I disabled Retrospect and Windows Server backup and that makes no difference.
If the error was caused by the backup, you can ignore the error or schedule the replication to stop when the backup processing.
Following link for your reference:
https://social.technet.microsoft.com/Forums/en-US/af89b4e2-ad28-4978-8355-3973cc476dfe/dfs-replication-issue-in-secondary-ad?forum=winserverDS

Maybe DFS is stuck