I just suddenly started having issues using VPN Manager for OpenVPN on mine as well. My issue just started yesterday. Booted libreelec, VPN manager tried to authenticate through Nord, but then it tells me "Could not authenticate with VPN provider. Please check user name and password and try again." I'm not sure where the issue is yet, because this happens on 11.0 and 12.0 nightlies. It was fine on both of those Friday/Saturday.
I know it isn't my username and password, because I as well am logged on through my laptop. I've changed 0 settings in the app in Kodi for both 11.0 and 12.0. I also tried to update the add-on, but there are no updates. I'm using the add-on from Zomboided which is 7.0.1.
It just randomly stopped working. I'll test it later today. Unfortunately I don't have another VPN provider to try with to see if it's a Nord issue or not.
This thread has been dead for a while but for those like me who might stumble upon it later and haven't found an answer yet, I want to share what the issue was when I faced the same situation.
Verify that you are using the username and password generated in CyberGhost when you created the profile for a linux/router device. Do NOT use your "normal" login credentials as those will lead to an authentication failure. Also, if you- like me- dabbled with the settings of the Zomboided VPN Service and changed the setting for "use openvpn and killall with sudo" to "Always" (or maybe "platform default", too- didn't check) undo that as that will also lead to an authentication failure but only if you have done the setup prior. Otherwise the setup wizard will tell you that killall and openvpn aren't available and the logs show that it was invoked with sudo which is unavailable on kodi.