Thanks for help. Apparently cipher AES-256-CBC was breaking the connection. When I used AES-128-CBC on client and server, after which I used command systemctl restart openvpn, everything seems to be working perfectly.
subzero79, thanks for your help!
P.S. I have removed what I should have not said in the previous message 
After trying this, I cannot connect to the server and get message:
read UDP: Unknown error (code=10054)
Checking the status on OMV shows that openvpn is running, but not possible to connect. I tried to disable and enable Openvpn from OMV page and I can connect to the server again, however all the settings are lost again. 
Is there other command to try restart openvpn?
I am not advanced linux or Openmediavault user. May I ask you to provide the terminal command needed for such restart of openvpn. I was not able to find any information on google.
Ok, so I have downloaded /etc/openvpn/server.conf file and added the mentioned line to the file. However every time I put it back there and restart OpenVPN plugin, changes are replaced with the original server.conf file. Any ideas on why i it happening?
So no forum members have encountered this issue before?
I am kind of lost, any help appreciated.
Hi, guys,
I receive error on OpenVPN clients when connecting to OpenVPN server which is on OMV 3 OS:
WARNING: INSECURE cipher with block size less than 128 bit (64 bit). This allows attacks like SWEET32. Mitigate by using a --cipher with a larger block size (e.g. AES-256-CBC).
Have anyone encountered this error before? Where could I change this cipher block size?
BR,
gedi
