Created attachment 746541 [details] vpn-error.png Last week I upgraded KDE to 5.23.1, including kde-plasma/plasma-nm package. Since then, whenever I click on my VPN connection in NM applet to connect, I'll get the error "Missing VPN plugin". I wasn't sure what was causing the problem because there were some changes to openconnect-related packages, too. However, it is working in GNOME. So I manually downgraded only kde-plasma/plasma-nm to v5.22.5 and my VPN is working again via plasma-nm.
Looks like PEBKAC. Because when I run into this problem I was expecting a problem between NM and openconnect so I did just restart these applications/services and not the complete KDE environment. During my bisect attempt I fully restart the system and I am now unable to reproduce the problem.