I am running KDE on Kubuntu 12.04. The network management and / or network monitor widget showed properly on the task bar and one day they disappeared. I can add them as desktop widgets, but not on the taskbar.

How can I restore the previous situation?

Thanks.
Benjamin

do you mean the systemtray? does it show there if you click the systemtray's "show hidden icons"?

Yes, I mean the system tray.
No, the Metwork Manager icon does not show in the hidden icons.
It does not even get a "tick" mark on selecting it in the widgets to choose from.

can you try as a new/different user

can you force a reinstall of the package plasma-widget-networkmanagement

If it is shown under the new user, please run "kbuildsycoca4 --noincremental" under your current user.

Before I could do anything, all widgets have disappeared from my panel AND the User Management icon has disappeared from the System Settings. I am now very confused...

PS: I will upgrade to Kubuntu 13.04 and that very probably will solve this matter.

It sounds like you have mixed package versions - please ensure all KDE packages come from the same repository and are of corresponding versions.
Mixing repositories is not recommended.

I upgraded to Kubuntu 13.04 and now the problem is solved.