Windows 7 Update is available - but not displayed

Today a yet unknown issue occurred to me with Windows 7 Updates.
It is different from questions I found researching for this topic. I didn't have a problem finding, downloading or installing the update.
I don't have an english system so I don't know the exact wording of the window I describe. I'll explain how to get there.

• The updater said there is an update available (the bubble and icon appeared in the tray bar).


• The Windows Update page in the Control Panel showed the download and now shows the update is downloaded.


• Control Panel's Windows Update page shows 1 important Update is available.



• Clicking on this text opens a window Chose updates to be installed where updates are shown with name and size and one can select what to install and what to skip.


• This window has one vertical tab saying Important (1) - there must be one important update.


• Now the strange part: This tab is selected but the list doesn't show an update.

I wonder what's happening and how can I fix it?

I opened this window multiple times and rebootet the PC. No change.

Finally I decided to install this update.
For those who want to know what it is. The updates history says at this day KB915597 - Windows Defender - was installed. There are 4 of these KB in my history, each with a different version number. Three of them already were installed on other days.