It caused by missing DH file, which is used in server side. It seems that you attempt to run server, but not client. If you indeed try to start OpenVPN client, could you please PM the OpenVPN configuration to me? So I can check it out.
Thanks! I did try to run the client. However by now I’ve resolved the problem by restoring the box to factory defaults so I don’t have that configuration any more.
What I suppose as a problem source is that I’ve installed an mbedTLS version of Transmission in that setup. Could its dependencies maybe somehow screw up the OpenSSL dependencies needed by OpenVPN?