The new update systray icon notifies me of available updates, clicking on it opens plasma discover which just shows LOADING... seemingly forever.
Once this happens I can't upgrade my system using apt, apt-get says there is a lock.

Only workaround I have found is to reboot the computer, switch to a virtual terminal before logging in KDE and use apt-get to upgrade the system.

Why replacing something that worked with something that's broken ?

I'm replying to myself to add that after a few reboots, now both discover and the update manager do not exhibit the problematic behaviour on this machine.

But in the mean time my grand father also running Debian Stretch and KDE has phoned me to tell me the update manager is showing exactly the symptoms I wrote about earlier. He also complained about discover search not finding software he knows is available in the repositories while giving many irrelevant results.

This bug seems to be an erratic one.