Network Problem with OpenVPN

FSL

For the past eight years the scenario below was working perfectly, including server upgrades during that period until about two weeks ago.
Cisco Router, a box running only an OpenVPN server, a Sonic Wall, switches, servers, clients.
The internet comes into the Cisco router, UDP on port 1194 goes to the OpenVPN server box everything else to the Sonic Wall and then onward to the rest of the network. (Despite what Sonic wall says it does not handle OpenVPN).
The OpenVPN server box has two NICs. NIC1 has a 10.0.x.x range and connects to the Cisco router using a fixed IP. NIC2 has a 192.168.x.x range (same as the rest of the network) and connects to the switch the network side of the Sonic Wall, again using a fixed IP. The NICs are not bridged.
Several remote locations connect in via NIC1 which connects to the Cisco and one machine at the centre connects via NIC2 which connects to the switch the network side of the Sonic Wall.
The VPNServer is set to allow client – client connectivity so the remote locations can connect to each other and to one machine at the centre and vice versa.
This as I said has been working without problems for about eight years or more, but suddenly it stopped working in so much as the centre machine could connect to the OpenVPN server but the remote machines timed out waiting for a response from the OpenVPN server. If the centre was unplugged from NIC2 the remote machines connect no trouble.
To eliminate any possible issue with the OpenVPN server box and the two NICs I set it up again on a spare box which was available, put in two brand new NICs. I connected two routers one to each NIC mirroring the IP address ranges of the original set up.
I then connected two laptops one to each NIC. Without the OpenVPN client running both could ping their respective NICs but not each other as it should be by design. When the OpenVPN client is started on each of the laptops it connects to the VPNServer and then, as it should be the two laptops can see each other.
Having ascertained that the OpenVPN server and its two NICs were behaving as expected I replaced the old VPNserver box with the new.
Unfortunately the same behaviour manifests itself. If the centre is connected to NIC2, NiC1 doesn’t behave as it should if NIC2 is disconnected the remote locations connect and can see each other.
It’s seemingly completely illogical but something must be causing the problem.
Does anybody have any clue or any suggestions? (Bridging the two NICs is out of the question).

White Heart Loon

Has anything changed on the sonicwall? Did you update it or make any changes to it lately?

FSL

No no changes to the Sonic Wall and the switch is the network side of the Sonic Wall.