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KDE (or X?) clipboard goes stupid

[kf5 5.38, plasma-workspace-5.10.5, Fedora 25]

My KDE/X clipboard goes astray sometimes. I Copy URL to clipboard from KMail, 
for example, and when I paste I get something from a day ago pasted.

Is there any way to reset that clipboard without restarting X or KDE? I'm not 
talking about Klipper, but the default clipboard.