I run into a bug where nm-connection-editor doesn't read the password out of the settings. And without password it doesn't save changes.
I hadn't recognized this first, so that I tested editing the setup as root. The console outputs leads me to a solution. After saving my changes as root, I no longer have access to the VPN. Even the entry isn't visible. Within the configuration file, I have found the following line. permissions=user:root:; I've changed it back to my login name, but it still doesn't work. How can I change the permissions back to me as owner?
