Bug 429885

note:
I report this to kded because the problem seems to be related to the way kde applications find out about network connection state and I can't find out how exactly this happens. 
kded seems to provide this information through dbus, so I chose it as "closest match".

SUMMARY
After several days of kmail not working, I noticed that akregator can't fetch feeds either.
This along with Bug #424245 and several hours of triyng stuff led to the following summary:

Applications not beeing able to get data from Network (with the "quality" of "Error Messages"):
Akonadi (KMail, KOrganizer) - "Connection established - Offline"
Akregator "Networking is not available" - the only useful :)
Konqueror (http(s) only - sftp still worked) something like "File not found" (yes! "file"!)

qdbus showed: back then (not working) | now after workaround (working)
$ qdbus org.kde.kded5 /modules/networkstatus org.kde.Solid.Networking.Client.status
0 | 4
$ qdbus org.kde.kded5 /modules/networkstatus org.kde.Solid.Networking.Service.networks
{} | SolidNetwork

Configuration: 
I was using ifupdown | netplan.io (now that I uninstalled netplan, I'm not sure the system was actually using it...)


Workaround that fixed everything:
1. uninstall netplan.io
2. install network-manager & plasma-nm
3. make network-manager manage wired connections
4. configure wired connection in plasma-nm


the whole problem started only recently (probably after updating to 5.76.0)

... I don't want network-manager, because there is only one dhcp-wired-connection and no need to ever change network configuration

STEPS TO REPRODUCE
1. uninstall network-manager

OBSERVED RESULT
no http for kde-apps

EXPECTED RESULT
no problems

SOFTWARE/OS VERSIONS
Distro: KDE Neon (focal) 
KDE Plasma Version: 5.20.3
KDE Frameworks Version: 5.76.0
Qt Version: 5.15.1

ADDITIONAL INFORMATION
This is related to #322602 and (at least my version of) #424245
Non-KDE software was not affected (tested with vivaldi and the qt http example)
It is not a user configuration problem - I had the same problems with a fresh user