I had a working DCO instance. After Upgrade 24.7.12 -> 25.1 the connection is established, but no traffic. Regular reconnects due to keepalive parameter.
Even pings to the serverside IP of the tunnel get no answers. Similar to Ticket #8178 (https://github.com/opnsense/core/issues/8178), but without clustering.
Client is openVPN 2.6.13 on windows (which worked with opnSense 24.7.12).