There was a moment, when KDE asked me whether I want to use sticky keys. It was an error, that I pressed Yes.

Now, if I hold Shift for some time, it sticks. To go back to lower case in letters I am to press Shift again.

How can I disable this behaviour?

systemsettings> accessibility> modifier keys. You can also turn off the gestures that activate them from the "activation gestures" tab.

Thanks, this works.