Comment 6 for bug 1577918

> installing "muon" may solve the problem, for me it worked.

The same for me; a quit from Discover then in a terminal window,

    sudo apt-get install muon

– after which, Discover began discovering what was sought.

Related
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http://kubuntu.org/news/kubuntu-16-04-lts-release-anouncement/

> Our new software center: **Plasma Discover** brim-full of software to choose from.

https://forum.kde.org/viewtopic.php?f=225&t=133288

> Issues with "Discover" (software utility) in Kubuntu 16.04

http://askubuntu.com/q/765130/25036 is now linked to a duplicate question http://askubuntu.com/q/761824/25036 with one answer but not (yet) the suggestion to simply install

muon